Search

CD4+ T cells and CD4+ chimeric antigen receptor (CAR) T cells display highly variable antitumor activity in preclinical models and in patients; however, the mechanisms dictating how and when CD4+ T cells promote tumor regression are incompletely understood. With the help of functional intravital imaging, we report that interferon (IFN)-γ production but not perforin-mediated cytotoxicity was the dominant mechanism for tumor elimination by anti-CD19 CD4+ CAR T cells. Mechanistically, mouse or human CD4+ CAR T-cell-derived IFN-γ diffused extensively to act on tumor cells at distance selectively killing tumors sensitive to cytokine-induced apoptosis, including antigen-negative variants. In anti-CD19 CAR T-cell-treated patients exhibiting elevated CAR CD4:CD8 ratios, strong induction of serum IFN-γ was associated with increased survival. We propose that the sensitivity of tumor cells to the pro-apoptotic activity of IFN-γ is a major determinant of CD4+ CAR T-cell efficacy and may be considered to guide the use of CD4+ T cells during immunotherapy.

International audience; The regulation of cellular energy metabolism is central to most physiological and pathophysiological processes. However, most current methods have limited ability to functionally probe metabolic pathways in individual cells. Here, we describe SPICE-Met (Single-cell Profiling and Imaging of Cell Energy Metabolism), a method for profiling energy metabolism in single cells using flow cytometry or imaging. We generated a transgenic mouse expressing PercevalHR, a fluorescent reporter for cellular ATP:ADP ratio. Modulation of PercevalHR fluorescence with metabolic inhibitors was used to infer the dependence of energy metabolism on oxidative phosphorylation and glycolysis in defined cell populations identified by flow cytometry. We applied SPICE-Met to analyze T-cell memory development during vaccination. Finally, we used SPICE-Met in combination with real-time imaging to dissect the heterogeneity and plasticity of energy metabolism in single macrophages ex vivo and identify three distinct metabolic patterns. Functional probing of energy metabolism with single-cell resolution should greatly facilitate the study of immunometabolism at a steady state, during disease pathogenesis or in response to therapy.

Neutrophils form cellular clusters or swarms in response to injury or pathogen intrusion. Yet, intracellular signaling events favoring this coordinated response remain to be fully characterized. Here, we show that calcium signals play a critical role during mouse neutrophil clustering around particles of zymosan, a structural fungal component. Pioneer neutrophils recognizing zymosan or live Candida albicans displayed elevated calcium levels. Subsequently, a transient wave of calcium signals in neighboring cells was observed followed by the attraction of neutrophils that exhibited more persistent calcium signals as they reached zymosan particles. Calcium signals promoted LTB4 production while the blocking of extracellular calcium entry or LTB4 signaling abrogated cluster formation. Finally, using optogenetics to manipulate calcium influx in primary neutrophils, we show that calcium signals could initiate recruitment of neighboring neutrophils in an LTB4-dependent manner. Thus, sustained calcium responses at the center of the cluster are necessary and sufficient for the generation of chemoattractive gradients that attract neutrophils in a self-reinforcing process.

International audience; Cytotoxic T cells (CTLs) can eliminate tumor cells through the delivery of lethal hits, but the actual efficiency of this process in the tumor microenvironment is unclear. Here, we visualized the capacity of single CTLs to attack tumor cells in vitro and in vivo using genetically encoded reporters that monitor cell damage and apoptosis. Using two distinct malignant B-cell lines, we found that the majority of cytotoxic hits delivered by CTLs in vitro were sublethal despite proper immunological synapse formation, and associated with reversible calcium elevation and membrane damage in the targets. Through intravital imaging in the bone marrow, we established that the majority of CTL interactions with lymphoma B cells were either unproductive or sublethal. Functional heterogeneity of CTLs contributed to diverse outcomes during CTL-tumor contacts in vivo. In the therapeutic settings of anti-CD19 CAR T cells, the majority of CAR T cell-tumor interactions were also not associated with lethal hit delivery. Thus, differences in CTL lytic potential together with tumor cell resistance to cytotoxic hits represent two important bottlenecks for antitumor responses in vivo.

International audience; Chimeric antigen receptor (CAR) T cell therapy relies on the activity of a large pool of tumor-targeting cytotoxic effectors. Whether CAR T cells act autonomously or require interactions with the tumor microenvironment (TME) remains incompletely understood. Here, we report an essential cross-talk between CAR T cell subsets and the TME for tumor control in an immunocompetent mouse B cell lymphoma model of anti-CD19 CAR T cell therapy. Using single-cell RNA sequencing, we revealed substantial modification of the TME during CAR T cell therapy. Interferon-γ (IFN-γ) produced by CAR T cells not only enhanced endogenous T and natural killer cell activity but was also essential for sustaining CAR T cell cytotoxicity, as revealed by intravital imaging. CAR T cell-derived IFN-γ facilitated host interleukin-12 production that supported host immune and CAR T cell responses. Compared with CD8+ CAR T cells, CD4+ CAR T cells were more efficient at host immune activation but less capable of direct tumor killing. In summary, CAR T cells do not act independently in vivo but rely instead on cytokine-mediated cross-talk with the TME for optimal activity. Invigorating CAR T cell interplay with the host represents an attractive strategy to prevent relapses after therapy.

Background Interferon beta (IFNβ) has been prescribed as a first-line disease-modifying therapy for relapsing-remitting multiple sclerosis (RRMS) for nearly three decades. However, there is still a lack of treatment response markers that correlate with the clinical outcome of patients. Aim To determine a combination of cellular and molecular blood signatures associated with the efficacy of IFNβ treatment using an integrated approach. Methods The immune status of 40 RRMS patients, 15 of whom were untreated and 25 that received IFNβ1a treatment (15 responders, 10 non-responders), was investigated by phenotyping regulatory CD4+ T cells and naïve/memory T cell subsets, by measurement of circulating IFNα/β proteins with digital ELISA (Simoa) and analysis of ~600 immune related genes including 159 interferon-stimulated genes (ISGs) with the Nanostring technology. The potential impact of HLA class II gene variation in treatment responsiveness was investigated by genotyping HLA-DRB1, -DRB3,4,5, -DQA1, and -DQB1, using as a control population the Milieu Interieur cohort of 1,000 French healthy donors. Results Clinical responders and non-responders displayed similar plasma levels of IFNβ and similar ISG profiles. However, non-responders mainly differed from other subject groups with reduced circulating naïve regulatory T cells, enhanced terminally differentiated effector memory CD4+ TEMRA cells, and altered expression of at least six genes with immunoregulatory function. Moreover, non-responders were enriched for HLA-DQB1 genotypes encoding DQ8 and DQ2 serotypes. Interestingly, these two serotypes are associated with type 1 diabetes and celiac disease. Overall, the immune signatures of non-responders suggest an active disease that is resistant to therapeutic IFNβ, and in which CD4+ T cells, likely restricted by DQ8 and/or DQ2, exert enhanced autoreactive and bystander inflammatory activities.

Type I IFN can exert pro- and anti-inflammatory activities in the immune system. Here, we have investigated the mechanism by which IFN-α enhances early expression of the anti-inflammatory cytokine IL-10 in human CD45RA+CD4+ T cells. With the use of transcriptomic and biochemical approaches, we found distinct and combined contributions of the IFN and the TCR signaling pathways to the induction of STAT1/2/3 and the basic leucine zipper activating transcription factor-like (BATF) family members. Moreover, IFN-induced STAT3 phosphorylation was prolonged by the TCR response, whereas IFN-induced STAT2 phosphorylation was of long duration. With the use of RNA interference (RNAi), we identified STAT3 as the major actor and STAT2 as a contributor of the IFN action on IL-10. Upon TCR/IFN costimulation, STAT3 directly bound at the IL-10 conserved noncoding sequence (CNS)- 9, an enhancer element known to recruit BATF in CD4 T cells. The cosilencing of the 3 BATFs resulted in an overall reduction of IL-10 expression, but the promoting activity of IFN-α was retained. These results support the notion that the IFN action is indexed on BATF function and provide evidence for a cooperation between BATFs and STAT3, the latter being activated via early IFN and delayed TCR effects.

Type I interferons (IFNs) have the dual ability to promote the development of the immune response and exert an anti-inflammatory activity. We analyzed the integrated effect of IFN-α, TCR signal strength, and CD28 costimulation on human CD4⁺ T-cell differentiation into cell subsets producing the anti- and proinflammatory cytokines IL-10 and IFN-γ. We show that IFN-α boosted TCR-induced IL-10 expression in activated peripheral CD45RA⁺CD4⁺ T cells and in whole blood cultures. The functional cooperation between TCR and IFN-α efficiently occurred at low engagement of receptors. Moreover, IFN-α rapidly cooperated with anti-CD3 stimulation alone. IFN-α, but not IL-10, drove the early development of type I regulatory T cells that were mostly IL-10⁺ Foxp3⁻ IFN-γ⁻ and favored IL-10 expression in a fraction of Foxp3⁺ T cells. Our data support a model in which IFN-α costimulates TCR toward the production of IL-10 whose level can be amplified via an autocrine feedback loop.

International audience; Recognition of various HIV proteins by CD8 T cells from HIV-infected children was determined by two functional assays. First, using an Elispot assay, we show that 80% of patients recognized Gag, 77% recognized Pol, 61% recognized Env, 44% recognized Nef and 29% recognized Vif. Frequencies of Gag-, Pol-, and Env-specific IFN-gamma producing CD8 T cells were higher than frequencies of Nef and Vif-specific CD8 T cells. The poor recognition of Nef by ex vivo CD8 T cells was confirmed by CTL assays performed in HAART na? children: 25% of children had positive response against Nef versus 44, 63 and 62% for Env, Gag, and Pol, respectively. Memory Gag-specific CTL were positively correlated with age, whereas Nef-specific CTL were negatively correlated with age. The poor Nef-specific CD8 T cell response in HIV-infected children contrasts with dominance of Nef-specific responses in infected adults.

Toll-like receptors (TLRs) are considered an essential component of the innate immune system, initiating inflammatory responses following infection of the host. Humans have 10 functional TLRs, differing in their subcellular distributions and the microbial agonists they sense. The phylogenetically conserved TLR1-2-6 family is unique in that TLR1 and TLR6 form heterodimers with TLR2 to mediate signalling in response to agonists. Epidemiological genetic studies have identified several TLR variants that appear to influence susceptibility to infectious diseases, but the functional consequences of which remain largely unknown. Here, we assessed the functional impact of the TLR1-2-6 variants with altered amino acid sequences segregating naturally in the human population. We used an NF-κB reporter assay in TLR-transfected human embryonic kidney 293T cells stimulated with the corresponding TLR agonists. We found that among the 41 naturally occurring variants with amino acid alterations identified in the TLR1-2-6 family, 14 of them (five TLR1, four TLR2, and five TLR6 variants) displayed marked impairment of NF-κB activation. Most of these variants are present at very low population frequencies and are population-specific. These observations suggest that rare, nonsynonymous TLR mutations are likely to have deleterious effects on immune responses and may therefore contribute to complex susceptibility to infection at the population level. Hum Mutat 32:643–652, 2011. © 2011 Wiley-Liss, Inc.

Background One of the earliest activation events following stimulation of the T cell receptor (TCR) is the phosphorylation of the immunoreceptor tyrosine-based activation motifs (ITAMs) within the CD3-associated complex by the Src family kinase Lck. There is accumulating evidence that a large pool of Lck is constitutively active in T cells but how the TCR is connected to Lck and to the downstream signaling cascade remains elusive. Methodology/Principal Findings We have analyzed the phosphorylation state of Lck and Fyn and TCR signaling in human naive CD4+ T cells and in the transformed T cell line, Hut-78. The latter has been shown to be similar to primary T cells in TCR-inducible phosphorylations and can be highly knocked down by RNA interference. In both T cell types, basal phosphorylation of Lck and Fyn on their activatory tyrosine was observed, although this was much less pronounced in Hut-78 cells. TCR stimulation led to the co-precipitation of Lck with the transmembrane adaptor protein LAT (linker for activation of T cells), Erk-mediated phosphorylation of Lck and no detectable dephosphorylation of Lck inhibitory tyrosine. Strikingly, upon LAT knockdown in Hut-78 cells, we found that LAT promoted TCR-induced phosphorylation of Lck and Fyn activatory tyrosines, TCRζ chain phosphorylation and Zap-70 activation. Notably, LAT regulated these events at low strength of TCR engagement. Conclusions/Significance Our results indicate for the first time that LAT promotes TCR signal initiation and suggest that this adaptor may contribute to maintain active Lck in proximity of their substrates.

The family of type I interferon (IFN-I) exerts pro- and anti-inflammatory activities. We have previously shown that IFN-I potentiates expression of the anti-inflammatory cytokine, IL-10, in human naive CD4 T cells and promotes development of type 1 regulatory-like T cells. In order to understand how IFN-I potentiates T cell-receptor-induced IL-10 transcription, the mRNA levels of a broad set of transcription factors (TFs) potentially involved in the regulation of IL-10 were analyzed by RT-PCR. Of the ∼ 100 TFs studied, the Basic Leucine Zipper ATF-like (BATFs) were among the 12 TFs that showed an expression profile similar to that of IL-10 . The BATFs, which are members of the AP-1 family, have been reported to have positive transcriptional activity in lymphocytes and myeloid cells and are functionally redundant depending on the cytokine environment. We found that IFN-I alone rapidly increased BATF2 and BATF3 mRNA levels, and further enhanced TCR-induced BATF , BATF2 and BATF3 mRNA levels. Gene silencing assays of BATFs and other candidate IL-10 regulators by RNAi was conducted. Concurrent knockdown of the three BATFs or STAT3 resulted in markedly reduced IL-10 expression in response to TCR/IFN costimulation. These results indicate that BATFs are interferon-stimulated genes (ISGs) in human CD4 T cells and that BATFs may act in concert with STAT3 to regulate TCR/IFN-promoted IL-10 transcription.

International audience; Adaptor proteins positively or negatively regulate the T cell receptor for antigen (TCR) signaling cascade. We report that after TCR stimulation, the inhibitory adaptor downstream of kinase (Dok)-2 and its homologue Dok-1 are involved in a multimolecular complex including the lipid phosphatase Src homology 2 domain-containing inositol polyphosphate 5'-phosphatase (SHIP)-1 and Grb-2 which interacts with the membrane signaling scaffold linker for activation of T cells (LAT). Knockdown of LAT and SHIP-1 expression indicated that SHIP-1 favored recruitment of Dok-2 to LAT. Knockdown of Dok-2 and Dok-1 revealed their negative control on Akt and, unexpectedly, on Zap-70 activation. Our findings support the view that Dok-1 and -2 are critical elements of a LAT-dependent negative feedback loop that attenuates early TCR signal. Dok-1 and -2 may therefore exert a critical role in shaping the immune response and as gatekeepers for T cell tolerance.

HIV-specific CD8+ T lymphocytes participate in the control of viral replication in infected patients. These responses are of low intensity in young infants and are decreased by antiretroviral therapy. In the present study, we report on a recombinant Vaccinia virus (rVV)-based Elispot assay for the detection of HIV-specific CD8+ T cells immediately after isolation of peripheral blood mononuclear cells (PBMC). The rVV-based assay was highly sensitive; 48 out of 50 children had a positive response against the rVV encoding HIV Env-Gag-Pol antigen. Interferon-gamma was produced by CD8+ T cells, and CD14+/15+ cells were the main cell subset presenting antigens expressed by rVV. We observed that the cell input per well had a critical influence on the sensitivity of the assay. Results from the ex vivo Elispot assay correlated poorly with those of the 51Cr release assay performed after expansion of PBMC in vitro; thus, both assays gave information on different subsets and/or functions of the HIV-specific T cell response.

International audience; In the pediatric human immunodeficiency virus type-1 (HIV-1) infection, the presence of cytotoxic T lymphocytes (CTL) is associated with a slow progression to AIDS. The secretion of cytokines by CTLs may be critical in the control of viral infection. We used the combination of cell surface and intracellular staining to study the functionality of tetramer binding CD8+ T cells recognizing two HIV-1 immunodominant epitopes, in peripheral blood mononuclear cells from HIV-1-infected children. A fraction of tetramer positive CD8+ T cells produce cytokines (IFN-gamma, TNF-alpha) or chemokines (CCL4, CCL5) after ex vivo stimulation with the cognate peptide. There was a negative correlation between the plasma viral load and the percentage of CD8+ Tetramer Gag+ T cells secreting IFN-gamma. This is the first report in the context of pediatric HIV-1 infection showing that only a fraction of HIV-1-specific CD8+ T cells have the capacity to produce cytokines and chemokines implicated in their antiviral functions.

Introduction Type I interferons (IFNs) exert a complex role in the immune system as shown by their capacity to drive anti- and proinflammatory responses. Previous studies have highlighted contrasting effects of IFNs in the development of CD4 T cell adaptive immune response. We have tested the hypothesis that IFN α may modulate the expression of anti- and proinflammatory cytokines IL-10 and IFN γ according to intrinsic parameters of T cell activation, e.g. T cell receptor (TCR) stimulation strength and CD28 costimulation, and to IFN dose. Methods The effect of IFN α was analyzed in whole blood samples from healthy donors and in peripheral nai¨ve CD4 T cells that were stimulated through TCR/CD3- and CD28-specific antibodies. Cytokine transcription and secretion were quantified by qPCR and Elisa. The differentiation of IL-10 and IFN γ -producing T cell subsets from nai¨ve CD4 T cells was examined by intracellular cytokine staining and flow cytometry. Results IFN α enhanced the early expression of IL-10 in a synergistic manner with TCR stimulation and promoted the differentiation of nai¨ve CD4 T cells into IL-10-producing T cell subsets, characterized as being mainly IL-10+ IFN γ -Tr1 cells (Tr1-IFN). These cells were efficiently generated in the presence of IFN α , provided that suitable TCR signal was elicited either through the help of CD28 or, in its absence, through strong TCR/CD3 stimulation. Conclusion We provide evidence for a potent TCR/IFN crosstalk promoting IL-10 expression. Our data support a two-step model in which IFN α costimulates TCR for IL-10 induction, which may be further amplified via an autocrine positive feedback loop. These novel findings may help understand immunoregulatory functions of type I interferons.

Human immunodeficiency virus (HIV)-specific CD8+-T-cell responses are critical in restricting viral replication and altering the course of HIV infection (38, 48). Ex vivo cytotoxic T lymphocytes (CTL) are much less frequent in vertically infected children than in adults (9, 10, 41, 42, 46, 60). In contrast, after in vitro culture, memory CTL can be readily detected in HIV-infected children, with magnitude, breadth, and specificity similar to those observed in adults (9, 10, 41, 46). These memory CTL can be detected during the first weeks or months of life (10, 41, 56, 75), and the presence of HIV-specific CTL is associated with slower evolution toward disease (10). Complete suppression of viral replication following combined therapy is less frequent in children than in adults (40, 51, 52), and the kinetics of viral decrease is slower (49). However, children have a more active thymus function than adults (19, 44), which allows better lymphocyte regeneration following combined therapy (13, 69). In children treated with combined therapy before 3 months of age, and with undetectable viral load, neither HIV-specific antibodies nor HIV-specific CD4 or CD8 cells were detected (43). This contrasts with the beneficial effect of early treatment on preservation of HIV-specific T-cell immunity in adults (8, 50). New immunotherapeutic interventions are being developed for adults (8, 50). These treatments are of great interest for children due to observance problems and because complete viral suppression is more difficult to obtain in children than in adults. Therefore, it is crucial to characterize the dynamics of HIV-specific T cells in pediatric HIV infection. On encountering viral antigen, naive CD8+ T cells proliferate and differentiate into effector cells able to lyse infected cells and to secrete cytokines. As virus is cleared, most activated effector cells undergo apoptosis, but some survive and enter the memory pool that persists for long periods (76). Most memory cells are in a resting state, unable to secrete cytokine or to lyse infected cells, until reactivation on reexposure to viral antigen (61). During persistent infection, continuous stimulation of T cells may lead to dysfunction, anergy, or clonal exhaustion. In the absence of CD4+ T cells, exhaustion and anergization of CD8+ T cells is more rapid (78). The complex interplay between the antigen load, virus-specific CD8+-T-cell dynamics and function, and the immune status of the infected host has been illustrated by a number of studies of HIV-infected adults. In untreated chronically HIV-infected patients, CTL numbers are inversely correlated with the plasma viral load (5, 24, 25, 34, 54, 58). On the other hand, reduction of HIV replication by potent antiretroviral therapy is associated with a decline in HIV-specific CD8+ T cells (11, 55). Evidence that a fraction of HIV-specific CD8+ T cells are not functional was recently obtained (22, 31). Furthermore, loss of gamma-interferon (IFN-γ)-producing cells with persistence of tetramer binding cells was observed in subjects progressing to AIDS (32). CTL derived from the memory pool after in vitro expansion are easily detected by the standard 51Cr release assay in most HIV-infected children (9). In contrast, ex vivo-activated effector cytolytic cells are infrequently detected immediately after isolation (9). Cytokine synthesis following short-term ex vivo stimulation with the antigen can be measured using assays that are more sensitive than the chromium release assay and could be used to quantify the effector subset of HIV-specific CD8+ T cells (3, 18). Cytokine production can be measured at the single-cell level using the enzyme-linked immunospot (ELISPOT) technique, allowing direct calculation of T-cell frequencies (15). This technique is very sensitive and has been found to reliably detect CD8+ T cells in various human diseases (35, 63, 64, 74), including HIV infection (16, 36). The aim of the present work was to evaluate the use of the ELISPOT assay for the ex vivo study of HIV-specific CD8+ T cells from HIV-infected children. We chose to focus our initial effort on two immunodominant HLA-A∗0201-restricted HIV epitopes that are frequently recognized by infected children, as we showed previously using tetramers (66). In this cross-sectional study, HLA-A∗0201-positive HIV-infected children were systematically tested for the presence of HIV-specific CD8+ T cells using the ELISPOT assay. The frequencies of HIV-specific IFN-γ-producing CD8+ T cells were compared to the frequencies of HIV-specific CD8+ T cells measured by tetramer labeling, and relationships with biological parameters of HIV infection were investigated. The results from the ELISPOT and the tetramer assays were well correlated, but a comparison of the results from both assays suggests that a significant fraction of CD8+ T cells were unable to produce IFN-γ. Most importantly, the frequencies of ex vivo-activated HIV-specific CD8+-T-cell-mediated IFN-γ production were positively correlated with plasma HIV RNA, showing that this subset of antiviral CD8+ T cells is dependent upon continuous antigenic stimulation.

A previous study showed that, during the first year of life, the presence of cytotoxic T lymphocytes (CTLs) in human immunodeficiency virus (HIV)–infected children is associated with a lack of rapid progression to acquired immunodeficiency syndrome. The goal of the study was to address the role of CTLs in children who survived after age 5 years. Memory HIV-specific CTLs directed against Env, Gag, Nef, and Pol proteins were measured in a group of 47 highly active antiretroviral therapy–naive HIV-infected children. Both Gag- and Polspecific CTLs were positively correlated with CD4 + T cell counts. Gag-, Nef-, and Pol-specific CTLs were inversely correlated with virus load. The inverse correlation between virus load and Gag-specific CTLs was independent of CD4 + T cell counts. In conclusion, this study showed the beneficial role of HIV-specific CTLs in children who survived after age 5 years. The role of virus-specific cytotoxic T lymphocytes (CTLs) in the control of human immunodeficiency virus (HIV)andsimian immunodeficiency virus (SIV) replication has been established by a great number of studies. The elimination of CD8 T cells in rhesus macaques results in a dramatic increase in SIV load in both acute and chronic infection [1‐4]. The vaccination of monkeys capable of inducing SIV-specific cellular immune responses results in improved control of viral replication and longer survival after challenge [5]. In HIV-infected adults,acute viremia resolves with the appearance of CTLs, which precedes the detectable production of any neutralizing antibodies [6‐10]. The selection of virus mutants in vivo that are no longer recognized by CTLs was observed in both acute and late HIV infection, because virus levels increased [11‐13]. Long-term nonprogression is associated with the maintenance of high levels of both effector [14] and memory CTLs [15, 16]. Finally, the presence of effector CTLs against Gag [17], memory CTLs

We have previously shown the induction of humoral and cytotoxic responses specific for human immunodeficiency virus (HIV) and hepatitis B virus (HBV) antigens, following genetic immunization of rhesus macaques with a plasmid encoding both the third variable domain of the HIV-1 external envelope glycoprotein and the pseudo-viral particle of hepatitis B surface antigen (HBsAg) as presenting molecules. The DNA-immunized primates and two control animals were then challenged with a chimeric simian/human immunodeficiency virus (SHIV). They were all infected. Significant frequencies of SHIV specific cytotoxic T lymphocyte precursors (CTLp) were detected early in peripheral blood. But, in all DNA-immunized macaques, HBV envelope specific CTLp were detected during the primary infection, and they were correlated with the peak of SHIV viremia. Furthermore, HBV or SHIV specific cytotoxicity corresponded in part to CD8(+) T cells presenting a memory phenotype. Several mechanisms could account for this cellular response. But our results suggest that an expansion of memory cytotoxic CD8(+) cells, not restricted to SHIV specific effectors, could occur in peripheral blood during SHIV primary infection.