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Obesity is one of the leading health concerns in the United States. Studies from human and rodent models suggest that inherent differences in the function of brain motivation centers, including the nucleus accumbens (NAc), contribute to overeating and thus obesity. For example, there are basal enhancements in the excitability of NAc GABAergic medium spiny neurons (MSN) and reductions in basal expression of AMPA-type glutamate receptors in obesity-prone vs obesity-resistant rats. However, very little is known about the regulation of extracellular glutamate and GABA within the NAc of these models. Here we gave obesity-prone and obesity-resistant rats stable isotope-labeled glucose (

A number of machine learning methods can be employed in the prediction of suicide attempts. However, many models do not predict new cases well in cases with unbalanced data. The present study impro...

Two studies examined the effectiveness of the Unconscious Bias Juror (UBJ) video and instructions at reducing racial bias in Black and White mock-jurors’ decisions, perceptions, and counterfactual endorsement in a murder (Study 1; N = 554) and battery (Study 2; N = 539) trial. Participants viewed UBJ video or not. Then participants read pretrial instructions (general or UBJ), trial summary, and posttrial instructions (general or UBJ). In Study 1, juror race moderated the effect of defendant race on verdicts, culpability, and credibility. White, but not Black, jurors demonstrated greater leniency toward Black defendants for verdicts, culpability, and credibility. UBJ video moderated the effect of defendant race on murder counterfactual endorsement. Only when the video was absent was jurors’ counterfactual endorsement higher for the White versus Black defendant. This endorsement mediated the effect of defendant race on White jurors’ verdicts. In Study 2, White jurors were more lenient regardless of defendant race. Instructions and juror race moderated the video’s effect on credibility ratings. The video only influenced Black jurors’ credibility ratings. In conclusion, the debiasing interventions were ineffective in reducing racial bias in jurors’ verdicts. However, they do impact aspects of juror attribution and may be effective with modification.

Combination therapy with PD-1 blockade and IL-2 is highly effective during chronic lymphocytic choriomeningitis virus infection(1). Here we examine the underlying basis for this synergy. We show that PD-1 + IL-2 combination therapy, unlike PD-1 monotherapy, dramatically changes the differentiation program of the PD-1(+)TCF-1(+) stem-like CD8(+) T cells and results in the generation of transcriptionally and epigenetically distinct effector CD8(+) T cells that resemble highly functional effector CD8(+) T cells seen after an acute viral infection. The generation of these qualitatively superior CD8(+) T cells that mediate viral control underlies the synergy between PD-1 and IL-2. Our results show that the PD-1(+)TCF-1(+) stem-like CD8(+) T cells, also referred to as precursors of exhausted CD8(+) T cells, are not fate-locked into the exhaustion program and their differentiation trajectory can be changed by IL-2 signals. These virus-specific effector CD8(+) T cells emerging from the stem-like CD8(+) T cells after combination therapy expressed increased levels of the high affinity IL-2 trimeric (CD25, CD122, CD132) receptor. This was not seen after PD-1 blockade alone. Finally, we show that CD25 engagement with IL-2 plays an important role in the observed synergy between IL-2 cytokine and PD-1 blockade. Either blocking CD25 with an antibody or using a mutated version of IL-2 that does not bind CD25 but still binds CD122/CD132 almost completely abrogated the synergistic effects seen after PD-1 + IL-2 combination therapy. There is currently considerable interest in PD-1 + IL-2 combination therapy for patients with cancer(2,3) and our fundamental studies defining the underlying mechanisms of how IL-2 synergizes with PD-1 blockade should inform these human translational studies.

Differential interleukin-2 (IL-2) signaling and production are associated with disparate effector and memory fates. Whether the IL-2 signals perceived by CD8 T cells come from autocrine or paracrine sources, the timing of IL-2 signaling and their differential impact on CD8 T cell responses remain unclear. Using distinct models of germline and conditional IL-2 ablation in post-thymic CD8 T cells, this study shows that paracrine IL-2 is sufficient to drive optimal primary expansion, effector and memory differentiation, and metabolic function. In contrast, autocrine IL-2 is uniquely required during primary expansion to program robust secondary expansion potential in memory-fated cells. This study further shows that IL-2 production by antigen-specific CD8 T cells is largely independent of CD4 licensing of dendritic cells (DCs) in inflammatory infections with robust DC activation. These findings bear implications for immunizations and adoptive T cell immunotherapies, where effector and memory functions may be commandeered through IL-2 programming.

The cytokine IL-2 is critical for promoting the development, homeostasis, and function of regulatory T (Treg) cells. The cellular sources of IL-2 that promote these processes remain unclear. T cells, B cells, and dendritic cells (DCs) are known to make IL-2 in peripheral tissues. We found that T cells and DCs in the thymus also make IL-2. To identify cellular sources of IL-2 in Treg cell development and homeostasis, we used Il2FL/FL mice to selectively delete Il2 in T cells, B cells, and DCs. Because IL-15 can partially substitute for IL-2 in Treg cell development, we carried out the majority of these studies on an Il15−/− background. Deletion of Il2 in B cells, DCs, or both these subsets had no effect on Treg cell development, either in wild-type (WT) or Il15−/− mice. Deletion of Il2 in T cells had minimal effects in WT mice but virtually eliminated developing Treg cells in Il15−/− mice. In the spleen and most peripheral lymphoid organs, deletion of Il2 in B cells, DCs, or both subsets had no effect on Treg cell homeostasis. In contrast, deletion of Il2 in T cells led to a significant decrease in Treg cells in either WT or Il15−/− mice. The one exception was the mesenteric lymph nodes where significantly fewer Treg cells were observed when Il2 was deleted in both T cells and DCs. Thus, T cells are the sole source of IL-2 needed for Treg cell development, but DCs can contribute to Treg cell homeostasis in select organs.

International audience; Interleukin (IL)-2 is a pleiotropic cytokine that is necessary to prevent chronic inflammation in the gastrointestinal tract 1-4. The protective effects of IL-2 involve the generation, maintenance and function of regulatory T (T reg) cells 4-8 , and the use of low doses of IL-2 has emerged as a potential therapeutic strategy for patients with inflammatory bowel disease 9. However, the cellular and molecular pathways that control the production of IL-2 in the context of intestinal health are undefined. Here we show, in a mouse model, that IL-2 is acutely required to maintain T reg cells and immunological homeostasis throughout the gastrointestinal tract. Notably, lineage-specific deletion of IL-2 in T cells did not reduce T reg cells in the small intestine. Unbiased analyses revealed that, in the small intestine, group-3 innate lymphoid cells (ILC3s) are the dominant cellular source of IL-2, which is induced selectively by IL-1β. Macrophages in the small intestine produce IL-1β, and activation of this pathway involves MYD88-and NOD2-dependent sensing of the microbiota. Our loss-of-function studies show that ILC3-derived IL-2 is essential for maintaining T reg cells, immunological homeostasis and oral tolerance to dietary antigens in the small intestine. Furthermore, production of IL-2 by ILC3s was significantly reduced in the small intestine of patients with Crohn's disease, and this correlated with lower frequencies of T reg cells. Our results reveal a previously unappreciated pathway in which a microbiota-and IL-1β-dependent axis promotes the production of IL-2 by ILC3s to orchestrate immune regulation in the intestine. To determine whether IL-2 is constitutively required for the maintenance of T reg cells and immunological homeostasis in the intestine, we administered isotype-control or anti-IL-2 neutralizing antibodies every other day to adult mice for two weeks. Within this short time period, the neutralization of IL-2 promoted an enlargement of the spleen and mesenteric lymph nodes, and caused significant reductions of T reg cells and significant increases in the proliferation of CD4 + T cells throughout the gastrointestinal tract and associated lymphoid tissues, including the mesenteric lymph nodes, large intestine and small intestine (Extended Data Fig. 1a-g). Blockade of IL-2 resulted in significantly enhanced IFNγ production by CD4 + T cells in both the small and large intestine, as well as increased IL-17A production in the large intestine (Extended Data Fig. 1h-k). Previous studies have suggested that CD4 + T cells are the dominant cellular source of IL-2 1,2. Therefore, we generated mice with a lineage-specific deletion of IL-2 in T cells by crossing IL-2-floxed mice 10 with Lck cre mice. Lck cre Il2 f/f mice exhibited a complete loss of IL-2 protein staining in T cells, and we observed a significant reduction in the number of T reg cells, and an increase in CD4 + T cell proliferation and effector function in the mesenteric lymph nodes and large intestine (Extended Data Fig. 2a-g). By contrast, deletion of

Differential IL-2 signaling is implicated in memory CD8 T cell fates. Autocrine IL-2 production is a hallmark of polyfunctional, long-lived lymphoid memory cells. Whether the IL-2 signals perceived by CD8 T cells come from autocrine or paracrine sources, the timing of these signals, and their differential impact on the qualitative outcomes of CD8 T cell responses are not clearly understood. We used two distinct models of germline and conditional autocrine IL-2 ablation in post-thymic CD8 T cells to specifically dissect the requirement and timing of autocrine IL-2. Our studies show that autocrine IL-2 is uniquely required during primary CTL responses to program the key memory property of robust recall responses. Ablation of autocrine IL-2 in fully differentiated wild type memory cells immediately prior to rechallenge did not impact their recall expansion potential. Furthermore, we found that paracrine IL-2 could effectively supplant autocrine IL-2 for primary expansion, effector differentiation and memory maintenance, but not recall expansion. Likewise, our data highlight the redundancy of autocrine and paracrine IL-2 in optimally driving equivalent metabolic programs associated with effector and memory states. While CD4 licensing of DCs is needed for optimal programming of autocrine IL-2 in CD8 T cells, we found that the requirement for CD4 help is less critical for autocrine IL-2 production in the context of inflammatory infections. These findings have implications in adoptive T cell immunotherapies where expansion of engineered cells is largely driven by paracrine IL-2, which can potentially impact their long-term sentinel function.

Interleukin (IL)-2 is a pleiotropic cytokine that is necessary to prevent chronic inflammation in the gastrointestinal tract

The inhibitory receptor programmed cell death 1 (PD-1) plays a major role in functional exhaustion of T cells during chronic infections and cancer, and recent clinical data suggest that blockade of the PD-1 pathway is an effective immunotherapy in treating certain cancers. Thus, it is important to define combinatorial approaches that increase the efficacy of PD-1 blockade. To address this issue, we examined the effect of IL-2 and PD-1 ligand 1 (PD-L1) blockade in the mouse model of chronic lymphocytic choriomeningitis virus (LCMV) infection. We found that low-dose IL-2 administration alone enhanced CD8+ T cell responses in chronically infected mice. IL-2 treatment also decreased inhibitory receptor levels on virus-specific CD8+ T cells and increased expression of CD127 and CD44, resulting in a phenotype resembling that of memory T cells. Surprisingly, IL-2 therapy had only a minimal effect on reducing viral load. However, combining IL-2 treatment with blockade of the PD-1 inhibitory pathway had striking synergistic effects in enhancing virus-specific CD8+ T cell responses and decreasing viral load. Interestingly, this reduction in viral load occurred despite increased numbers of Tregs. These results suggest that combined IL-2 therapy and PD-L1 blockade merits consideration as a regimen for treating human chronic infections and cancer.

The molecular mechanisms responsible for the exquisite discrimination between self and nonself molecules have remained enigmatic despite intense investigation. However, with the availability of adequate amounts of anergic lymphocytes produced by double transgenic mice, large numbers of immature B cells from sublethaly irradiated, hematopoietically-synchronized mice, as well as critical gene-deleted mice, it has been possible for the first time to uncover plausible molecular mechanisms that lead to tolerance versus immunity. The Rel family of transcription factors is expressed at different stages of lymphocyte maturation and differentiation. C-Rel is not activated by immature lymphocytes, which undergo either anergy or apoptosis when triggered by antigen receptors, but c-Rel is activated in mature lymphocytes. Antigen receptor triggering induces c-Rel-dependent survival and proliferative genetic programs. In T cells, a critical c-Rel-dependent gene encodes the T-cell growth factor interleukin-2 (IL-2). Thus, T cells from c-Rel gene-deleted mice produce inadequate quantities of IL-2, which renders them immunocompromised and unable to mount normal T-cell proliferative and differentiative responses. In the face of absolute IL-2 deficiency from birth, severe, multiorgan autoimmunity gradually ensues. Also, with more subtle IL-2 deficiency, organ/tissue-specific autoimmune disease becomes evident. Accordingly, both c-Rel and IL-2 appear to be key molecules for tolerance versus immunity, and doubtless will become foci for continued investigation, as well as future therapeutic targets in autoimmune diseases.

Interleukin (IL)-2 has been studied as a treatment for HIV infection because it stimulates the proliferation of T cells and augments antiviral immune functions [1,2]. Two regimens have been used: intermittent IL-2 (5 days every 8 weeks, usually 4.5–9 million international units (mIU)/day) and ultra-low-dose (ULD) IL-2 (1.2 mIU/m2/day), which can be given daily for weeks or months [1]. In patients with chronic and early/acute HIV infection, neither regimen has conferred any health benefit when given with concurrent antiretroviral therapy (ART) [3–7], and in patients with chronic HIV infection, ULD IL-2 did not improve viral control after stopping ART [8]. However, treatment of acute/early HIV infection with daily ULD IL-2 and interruption of ART has not been studied. We treated one patient who had taken ART since 1 month after HIV seroconversion with daily ULD IL-2. After receiving IL-2, this patient’s viral load set point off highly active antiretroviral therapy (HAART) was less than 50 copies/ml as compared with 39 000 copies/ml when ART was stopped without IL-2. Further, after IL-2 was stopped, viral load did not rebound for 14 months without ART. A 29-year-old man tested HIV-positive by ELISA and western blot 1.5 months after a negative ELISA, and was treated 30 days later with zidovudine, lamivudine, and efavirenz. Viral load was undetectable for 4 years on ART (with minor changes in regimen), except for occasional blips (all

Studies over the past 50 years revealing the molecular events that promote normal T lymphocyte cycle competence and progression led to a detailed understanding of how cytokines function to regulate normal hematopoietic cell proliferation. During that same period, the molecular and genetic changes introduced by the Philadelphia chromosome in chronic myelogenous leukemia were unraveled, and these have led to an understanding of how mutations that constitutively activate normal cytokine signaling pathways can cause unregulated cell proliferation and malignant transformation. Based on the paradigm established by these data, it is inescapable that going forward, investigators will operate under the hypothesis that transformation of additional cells and tissues will have a similar pathogenesis.

The regulation of the tempo, magnitude, and duration of the immune response has been thought to reside solely with antigen for the past 50 years. However, with the discovery of the interleukins (ILs) 30 years ago, it became evident that these endogenous 'lymphocytotrophic hormones' provide the molecular mechanisms via classic hormone-receptor interactions. However, lacking in the hormonal regulatory capacity of the ILs were negative feedback mechanisms that functioned to switch off the positive driving force of the immune response, whether after antigen was cleared or when antigen persists, as with auto-antigens, tumor antigens, persistent infections, or allografts. Our recent experimental data, reviewed herein, exploring the T-cell antigen receptor (TCR) induction of the negative transcriptional regulator, forkhead box protein 3 (FOXP3), indicate that its expression is signaled by the T-cell growth factor IL-2. Once expressed, FOXP3 functions to restrict IL-2 expression in reaction to continued TCR stimulation. Thus, IL-2 regulates it own levels via a FOXP3-mediated negative feedback loop. In contrast, we found no evidence that FOXP3(+) cells actively suppress IL-2 expression, thereby failing to support the notion that such cells regulate potential effector cells.

SUMMARY One potentially promising strategy to control chronic infections such as human immunodeficiency virus, hepatitis B virus, and hepatitis C virus is therapeutic vaccination, which aims to reduce persisting virus by stimulating a patient's own antiviral immune responses. However, this approach has fallen short of expectations, because antiviral T cells generated during chronic infections often become functionally exhausted and thus do not respond properly to therapeutic vaccination. Therefore, it is necessary to develop a therapeutic vaccine strategy to more effectively boost endogenous T-cell responses to control persistent viral infections. Studies to elucidate the cause of impaired T-cell function have pointed to sustained inhibitory receptor signaling through T-cell expression of programmed death 1 (PD-1). Recently, another inhibitory molecule, cytotoxic T lymphocyte antigen 4 (CTLA-4), and also an immunosuppressive cytokine, interleukin 10 (IL-10), have been reported to be potential factors of establishing immune suppression and viral persistence. Blocking these negative signaling pathways could restore the host immune system, enabling it to respond to further stimulation. Indeed, combining therapeutic vaccination along with the blockade of inhibitory signals could synergistically enhance functional CD8(+) T-cell responses and improve viral control in chronically infected mice, providing a promising strategy for the treatment of chronic viral infections. Furthermore, not only the ablation of negative signals but also the addition of stimulatory signals, such as interleukin 2 (IL-2), might prove to be a potentially promising strategy to augment the efficacy of therapeutic vaccination against chronic viral infections.

HIV-associated neurocognitive disorders (HAND) still occur in approximately 50% of HIV patients, and therapies to combat HAND progression are urgently needed. HIV proteins are released from infected cells and cause neuronal damage, possibly through mitochondrial abnormalities. Altered mitochondrial fission and fusion is implicated in several neurodegenerative disorders. Here, we hypothesized that mitochondrial fission/fusion may be dysregulated in neurons during HAND. We have identified decreased mitochondrial fission protein (dynamin 1-like; DNM1L) in frontal cortex tissues of HAND donors, along with enlarged and elongated mitochondria localized to the soma of damaged neurons. Similar pathology was observed in the brains of GFAP-gp120 tg mice. In vitro, recombinant gp120 decreased total and active DNM1L levels, reduced the level of Mitotracker staining, and increased extracellular acidification rate (ECAR) in primary neurons. DNM1L knockdown enhanced the effects of gp120 as measured by reduced Mitotracker signal in the treated cells. Interestingly, overexpression of DNM1L increased the level of Mitotracker staining in primary rat neurons and reduced neuroinflammation and neurodegeneration in the GFAP-gp120-tg mice. These data suggest that mitochondrial biogenesis dynamics are shifted towards mitochondrial fusion in brains of HAND patients and this may be due to gp120-induced reduction in DNM1L activity. Promoting mitochondrial fission during HIV infection of the CNS may restore mitochondrial biogenesis and prevent neurodegeneration.

This Research Topic was conceived to provide a venue for investigators to document their critical contributions to our understanding of the cellular and molecular mechanisms that provide our remarkable immune system the capacity to protect us from environmental insults while simultaneously remaining unreactive to our internal molecular milieu. Although the scientific literature gives one a history of what happened, when it happened, and who were responsible, it fails to capture precisely how things came together, and why some investigators were successful, while others working contemporaneously failed. Thus, seminal contributors were asked to recount the various aspects of their experiments and people who were instrumental in making the progress that moved our understanding forward. Looking back over the brief history of immunology, a discipline that arose after the pioneering approaches of Edward Jenner introduced the smallpox vaccine in 1798 (1) and Louis Pasteur catapulted immunology into universal awareness 100 years later (2–4), the science has only become mature within the past 50 years. For almost 100 years after Pasteur, experimentalists focused on observations of the reactions of whole experimental animals or humans to the administration of putative antigenic substances. For the first time, around 1960, it was appreciated that lymphocytes are the cells that mediate the immune reaction (5–7), and experimentation moved for the first time from in vivo to in vitro, which allowed one to manipulate and investigate an immune reaction of cell populations “outside of the black box.” During the 1960s, various techniques were improved so that it was possible to discern that several different types of cells cooperated to ultimately generate a measurable immune response, usually monitored by the appearance of antibody-forming cells (AFCs) (8, 9). As detailed in this Research Topic, by the 1970s, experiments culminated in the demonstration that two distinct types of lymphocytes existed, termed thymic-derived cells (T cells) and bone marrow-derived cells (B cells), which generate AFCs. Furthermore, a third type of cell derived from myeloid cells, termed an antigen-presenting cell (APC), also played a role. Evidence was also presented that there are at least two distinct subsets of T cells. Moreover, investigators detected activities in culture supernatants of activated lymphocyte populations that seemed to enhance or suppress the generation of AFCs as well as the proliferation of various lymphocytes. However, the molecular basis of these activities remained enigmatic and essentially unapproachable, given the experimental biochemical methods then available. Four new and novel experimental methods were introduced in the 1970s that revolutionized all of biological sciences, which enabled a further reduction from cells to molecules, and led to the discipline that now can be recognized as molecular immunology. In 1972, the flow cytometer introduced a new method to identify and isolate individual cells present in cell populations (10). Also, genetic engineering approaches enabled investigators to identify and isolate complimentary DNA molecules encoding gene products, which then allowed the ready determination of their primary structures, and provided a means to generate essentially unlimited quantities of critical proteins (11). Third, the advent in 1975 of the capacity to isolate and clone individual AFCs, which could produce unlimited quantities of monoclonal antibody molecules (12), could then be used to identify and isolate both individual cells using the flow cytometer, but also new reagents that could be used to isolate and purify individual protein molecules from complex mixtures. The fourth critical technical advance, which like monoclonal antibodies was also special to immunology, was the creation in 1979 of the methods to select, clone, and grow the functional progeny of individual T cells (13). This advance, for the first time, allowed one to circumvent the tremendous heterogeneity of individual cells within T cell populations so that the molecules responsible for antigen recognition, histocompatibility restriction, and the molecular mechanisms underlying T cell function, including T cell help of antibody production and T cell mediated cytolysis, could be uncovered. The contributions comprising this compilation of the stories about how the transition from experiments on whole living organisms to cell populations to individual cells and finally to homogeneous molecules represent a unique aspect of scientific communication, in that they tell the behind the scenes dramas that are usually left out of the scientific literature. This Research Topic tells how science gets done, and what the people are like who actually do it. I hope that you enjoy the stories.

Although we had successfully created antigen-specific cytolytic T lymphocyte lines (CTLLs) using conditioned media as a source of growth-promoting factors, we had no effective method to determine the relative activities of different batches of conditioned media. Therefore, it was crucial to create a quantitative assay for the activity we termed “T cell growth factor” (TCGF). Fortunately, Torgny Fredrickson and I had already created a bioassay for the red blood cell growth factor, erythropoietin (EPO), using murine fetal liver cells that are enriched for EPO-responsive precursors (1, 2). Thus, patterned on the EPO bioassay, it was straightforward to construct a similar assay for TCGF using as target cells our long-term CTLL. The critical elements of the assay were (1) a low density of CTLL target cells and (2) serial twofold dilutions of conditioned media samples, thereby establishing a dose–response curve that allowed comparison of separate conditioned media (3). Of note was the observation that the curve was symmetrically sigmoid when the linear responses of tritiated thymidine incorporation were plotted vs. the logarithm of the conditioned media dilutions. We arbitrarily assigned 1.0 U/mL that yielded 50% of maximal growth promotion at a dilution of 1:10. This assay represented the first ever quantitative bioassay for a lymphokine. Thus, armed with a rapid, quantitative bioassay, we next sought to generate T cell clones derived from our antigen-specific CTLL so that we could assess the potential problem of target cell heterogeneity. We tried two established cloning methods: (1) dilute cell suspensions seeded into soft agar containing TCGF-conditioned media and (2) limiting dilution (0.03–0.01 cells/well) in microtiter plates containing TCGF-conditioned media. The limiting dilution technique in suspension culture worked very well, yielding 67–100% plating efficiency. This was the first description of monoclonal antigen-specific cytolytic T cells (4). Accordingly, T cell clones permitted an unambiguous interpretation that TCGF was acting directly on cloned T cells and not indirectly through an intermediate cell type, e.g., an APC. We submitted our manuscript to Nature, which again rejected it without review [see Ref. (5)], so that we immediately reformatted it and sent it to the J. Exp. Med., which accepted it without changes, so much for non-scientist journalists (Nature) vs. peer scientists (JEM) making informed editorial decisions (6). Other investigators rapidly adopted these cloning methods, in that they not only allowed for separation of the cell clones, but also could be used to grow large numbers of progeny, which could be used for both biological and molecular characterizations. The ability to create monoclonal functional T cells was as transformative for T cells as monoclonal antibodies were for B cells. The foregoing results of these studies on the various mitogenic activities in conditioned media pointed to the overwhelming need to identify the molecules responsible for the bioactivities. In addition, the molecular mechanisms whereby the mitogenic activities interacted with their target cells loomed as a huge overriding question. Thus, armed with the quantitative TCGF bioassay, analytical biochemical experimental approaches yielded results consistent with a single, small protein (~15.5 kDa; pI = 8.2) as solely promoting the biological response (7). This was an important finding, in that it meant that further purification of the molecule responsible for the activity would be straightforward, whereas if several molecules cooperated to produce the activity, purification of each component would be difficult. These biochemical approaches permitted the separation and purification of enough biosynthetically radiolabeled TCGF to permit classic hormone binding assays, which revealed that radiolabeled TCGF-binding sites expressed all of the characteristics of true hormone receptors, i.e., the binding was restricted to TCGF-responsive cells, there was a lack of competition by other growth factors and hormones, the binding was of very high affinity, and there was a close correlation between the TCGF concentrations that bound to cells and those that mediated the proliferative response (8). These data all supported the conclusion that the binding site detected was on the receptor through which the biological effects of TCGF are initiated. This report was the first to demonstrate and characterize a cytokine receptor, and consequentially became the prototype for the identification and characterization of all subsequent cytokine receptors involved in regulating immune and inflammatory responses. Moreover, the radiolabeled TCGF binding assay was absolutely instrumental in the identification of the first monoclonal antibody reactive with a cytokine receptor molecule (9). These experimental approaches allowed us to calculate the Specific Activity of IL-2 for the first time, so that 1.0 U/mL = 150 ng/mL, which indicated that we would need to start with several liters of conditioned media to concentrate and purify adequate IL-2 protein to immunize mice with microgram amounts of TCGF protein and ultimately to develop the first monoclonal antibodies reactive with a lymphokine molecule (10). Prior to the development of quantitative bioassays and radioreceptor assays, investigators had tested only one dilution of a sample, e.g., a 1:2 or a 1:4, so that they could not quantify the amount of activity and relate it to a measured protein concentration. Thus, attempts were made to purify cytokine molecules with only 10–100 mL of starting conditioned media. In this article, we detailed the critical experimental approaches and advances that led to our success in generating monoclonal antibodies reactive with human TCGF, so that others might follow. Through rigorous biochemical analytic methods, it was possible to prove that the monoclonal antibodies were useful to immunoaffinity purify IL-2 molecules to homogeneity in milligram quantities from multiple liters of conditioned media that then could be used for unambiguous molecular and biological characteristics of the first interleukin molecule to be identified. Thus, with these new and novel cellular and molecular reagents, we could proceed to experiments that had never been done before.

Having created unique and novel cellular and molecular reagents, we could approach questions that had perplexed investigators interested in cell growth for over 50 years, i.e., the basis for variable cell cycle transit times of individual cells among genetically homogeneous cell populations (1). Studies of all cell populations, both prokaryote and eukaryote, revealed that within a cell population, the cell cycle times of individual cells follow a normal distribution when examined as a function of the division rate (the rate-normal distribution). Thus, some cells proliferate slowly while others proliferate faster, with most cells distributed about the mean on a log-linear plot. Prior to the discovery of the IL-2 molecule (2) and IL-2 receptors (IL-2R) (3), T cell populations were known to follow this rate-normal distribution, common to all living cells. Mathematical analysis of proliferating cell populations indicated that individual cell variability in cell cycle transit times was stochastic, depending upon a “hidden variable.” However, once the “hidden” molecular variables of T cell cycle proliferation had been identified (the IL-2 concentration, IL-2R density, affinity of the IL-2/IL-2R interaction, and the duration of the IL-2/IL-2R interaction), experiments could be preformed for the first time revealing that as long as these crucial characteristics of T cell cycle progression were known, then the variability of cell cycle transit times were entirely predictable and deterministic, not probabilistic or left to chance. These experiments and approaches were possible because of painstaking attention to the creation of critical cell clones and homogeneous purified IL-2 molecules, as well as the development of the radiolabeled IL-2 binding assay, and monoclonal antibodies reactive with both IL-2 and its receptor. Moreover, the employment of the flow cytometer allowed us to proceed beyond studies of cell populations to quantify the intermolecular interactions of individual cells for the first time. Doreen Cantrell, a postdoctoral fellow skilled in flow cytometry, was critical to our experimental approach. Because the growth characteristics of all known cell populations are identical to those of T cell populations, it followed that individual cells of all other cell populations would demonstrate the same type of molecular determinants. Thus, the title of this article: “The interleukin-2 T cell system: A new cell growth model,” a new universal paradigm in cell and molecular biology (1). The significance of these findings was obvious. As cells of all tissues, especially of metazoans, have identical growth characteristics of T cells, it followed that the cells of all metazoans are regulated in the same molecular fashion, i.e., cells are directed to undergo all-or-none (quantal) cell fate decisions by critical molecular concentrations. Furthermore, as all malignant cells arise from a single cell, so that all malignancies are clonal in origin, a gain of function mutation of one of the genes encoding the molecular determinants of cell cycle progression could obviate the strict cytokine/receptor/signaling pathways normally controlling the decision to divide, thereby resulting in a neoplastic cell growth (4). In other words, a critical “driver mutation” can put the cell on “autopilot.” From the viewpoint of the immune system, Burnet’s “Clonal Selection Theory,” derives from the observation that antigen-reactive clones of immune cells are selected by antigen, but subsequent to selection, each clone undergoes a proliferative clonal expansion (5). It is this clonal proliferative expansion that is determined by the molecular parameters discovered in this series of experiments focused on IL-2 and T cells. Accordingly, the regulatory cells and other cytokine molecules that influence the concentrations of the IL-2 molecules, their receptors, and the molecules of their signaling pathways ultimately determine the tempo, magnitude, and duration of immune responses. That is, internally derived hormones and their receptors regulate the immune system, just as every other bodily system is regulated, and immunity is not regulated solely by environmental antigens from without, a previous central dogma of immunology. For immunologists, it is especially noteworthy that the lymphocyte antigen receptors function in an identical fashion to convert TCR signals to a digital control of cytokine gene expression (6–10). These findings and considerations, led me to propose “The Quantal Theory of Immunity” (11–14). Thus, the immune system is regulated at the systemic (population) level by the number of clones responding and the extent of the proliferative expansion of each clone as originally stated by Burnet. However, at the level of individual cells, participation in an immune response is regulated in a quantal (all-or-none) fashion, and this quantal decision is determined by the absolute number of intermolecular interactions, which the cells somehow “count.” Once the crucial number has been surpassed, the cell responds in a quantal fashion. It goes without mentioning that all of the molecules and cells that participate in an immune response obey identical principles.

IL-2 is a cytokine that functions as a growth factor and central regulator in the immune system and mediates its effects through ligand-induced hetero-trimerization of the receptor subunits IL-2Rα, IL-2Rβ, and γc. Here, we describe the crystal structure of the trimeric assembly of the human IL-2 receptor ectodomains in complex with IL-2 at 3.0 Å resolution. The quaternary structure is consistent with a stepwise assembly from IL-2/IL-2Rα to IL-2/IL-2Rα/IL-2Rβ to IL-2/IL-2Rα/IL-2Rβ/γc. The IL-2Rα subunit forms the largest of the three IL-2/IL-2R interfaces, which, together with the high abundance of charge–charge interactions, correlates well with the rapid association rate and high-affinity interaction of IL-2Rα with IL-2 at the cell surface. Surprisingly, IL-2Rα makes no contacts with IL-2Rβ or γc, and only minor changes are observed in the IL-2 structure in response to receptor binding. These findings support the principal role of IL-2Rα to deliver IL-2 to the signaling complex and act as regulator of signal transduction. Cooperativity in assembly of the final quaternary complex is easily explained by the extraordinarily extensive set of interfaces found within the fully assembled IL-2 signaling complex, which nearly span the entire length of the IL-2Rβ and γcsubunits. Helix A of IL-2 wedges tightly between IL-2Rβ and γcto form a three-way junction that coalesces into a composite binding site for the final γcrecruitment. The IL-2/γcinterface itself exhibits the smallest buried surface and the fewest hydrogen bonds in the complex, which is consistent with its promiscuous use in other cytokine receptor complexes.

Exactly how the immune system discriminates between all environmental antigens to which it reacts vs. all self-antigens to which it does not, is a principal unanswered question in immunology. As set forth in this review, because of the advances in our understanding of the immune system that have occurred in the last 50 years, for the first time it is possible to formulate a new theory, termed the "Quantal Theory of Immunity", which reduces the problem from the immune system as a whole, to the individual cells comprising the system, and finally to a molecular explanation as to how the system behaves as it does.

IL-2, the first cytokine discovered with T cell growth factor activity, is now known to have pleiotropic effects on T cells. For example, it can promote growth, survival, and differentiation of Ag-selected cells, or facilitate Ag-induced cell death of T cells when Ag persists, and in vivo, it is thought to contribute to the regulation of the size of adaptive T cell response. IL-2 is deficient in HIV-1 infection and has been used in the management of HIV-1-infected individuals undergoing antiretroviral therapy. In this study, we investigated how continuous low-dose IL-2 affected the CD4+ and CD8+ T cell response induced by two inoculations of a canarypox recombinant SIV-based vaccine candidate in healthy macaques chronically infected with SIVmac251. These macaques had normal levels of CD4+ T cells at the beginning of antiretroviral therapy treatment. Vaccination in the presence of IL-2 significantly augmented Gag-specific CD8+ T cell responses, but actually reduced Gag-specific CD4+ T cell responses. Although IL-2 at low doses did not change the overall concentration of circulating CD4+ or CD8+ T cells, it expanded the frequency of CD4+CD25+ T cells. Depletion of the CD4+CD25+ T cells in vitro, however, did not result in a reconstitution of Gag-specific CD4+ responses or augmentation of SIV-specific CD8+ T cell responses. Thus, we conclude that the decrease in virus-specific CD4+ T cell response may be due to IL-2-promoted redistribution of cells from the circulation, or due to Ag-induced cell death, rather than suppression by a T regulatory population.

A cohort of rhesus macaques with long-standing SIVmac251 infection (≥5 mo) was treated with continuous antiretroviral therapy (ART). A group of eight macaques was vaccinated with or without simultaneous administration of low dose IL-2 with the highly attenuated poxvirus vector (NYVAC) vaccine candidate expressing the SIVmac structural gag-pol-env (gpe) genes and a novel chimeric fusion protein derived from the rev-tat-nef (rtn) regulatory genes. Control groups consisted of mock-vaccinated macaques or animals treated only with IL-2. Vaccination significantly expanded both virus-specific CD4+ and CD8+ T cell responses, and IL-2 further increased the vaccine-induced response to an immunodominant Gag epitope. Following antiretroviral treatment interruption, the viral set point was significantly lower in vaccinated than in control macaques for at least 4 consecutive mo, and viral containment was inversely correlated with vaccine-induced, virus-specific CD4+ and CD8+ T cell responses. These data provide the proof of concept that therapeutic vaccination before cessation of ART may be a feasible approach in the clinical management of HIV-1 infection.