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Biological therapies only benefit one-third of patients with Crohn's disease (CD). For this reason, a deeper understanding of the mechanisms by which biologics elicit their effect on intestinal mucosa is needed. Increasing evidence points toward the involvement of long noncoding RNAs (lncRNAs) in the pathogenesis of CD, although their role remains poorly studied. We aimed to characterize lncRNA profiles in the ileum and colon from CD patients and evaluate the effect of anti-TNF-α treatment on their transcription. Terminal ileum and left colon samples from 30 patients (active CD = 10, quiescent CD = 10, and healthy controls (HCs) = 10) were collected for RNA-seq. The patients were classified according to endoscopic activity. Furthermore, biopsies were cultured with infliximab, and their transcriptome was determined by Illumina gene expression array. A total of 678 differentially expressed lncRNAs between the terminal ileum and left colon were identified in HCs, 438 in patients with quiescent CD, and 468 in patients with active CD. Additionally, we identified three new lncRNAs in the ileum associated with CD activity. No differences were observed when comparing the effect of infliximab according to intestinal location, presence of disease (CD vs. HC), and activity (active vs. quiescent). The expression profiles of lncRNAs are associated with the location of intestinal tissue, being very different in the ileum and colon. The presence of CD and disease activity are associated with the differential expression of lncRNAs. No modulatory effect of infliximab has been observed in the lncRNA transcriptome.

Inflammatory bowel disease (IBD) is a chronic condition which includes ulcerative colitis (UC) and Crohn's disease (CD), the origins of which are not yet fully understood. Both conditions involve an exacerbated immune response in the intestinal tract, leading to tissue inflammation. Dendritic cells (DCs) are antigen-presenting cells crucial for maintaining tolerance in the gastrointestinal mucosa. Previous research has indicated that DC recruitment to the intestinal mucosa is more pronounced in individuals with IBD, but the specific mechanisms governing this migration remain unclear. This study aimed to assess the expression of various homing markers and the migratory abilities of circulating DC subsets in response to intestinal chemotactic signals. Additionally, this study examined how golimumab and ustekinumab impact these characteristics in individuals with IBD compared to healthy controls. The findings revealed that a particular subset of DCs known as type 2 conventional DCs (cDC2) displayed a more pronounced migratory profile compared to other DC subsets. Furthermore, the study observed that golimumab and ustekinumab had varying effects on the migratory profile of cDC1 in individuals with CD and UC. While CCL2 did not exert a chemoattractant effect on DC subsets in this patient cohort, treatment with golimumab and ustekinumab enhanced their migratory capacity towards CCL2 and CCL25 while reducing their migration towards MadCam1. In conclusion, this study highlights that cDC2 exhibits a heightened migratory profile towards the gastrointestinal mucosa compared to other DC subsets. This finding could be explored further for the development of new diagnostic biomarkers or the identification of potential immunomodulatory targets in the context of IBD.

Inflammatory bowel disease (IBD) is an idiopathic and chronic disorder that includes ulcerative colitis (UC) and Crohn's disease (CD). Both diseases show an uncontrolled intestinal immune response that generates tissue inflammation. Dendritic cells (DCs) are antigen-presenting cells that play a key role in tolerance maintenance in the gastrointestinal mucosa. Although it has been reported that DC recruitment by the intestinal mucosa is more prominent in IBD patients, the specific mechanisms governing this migration are currently unknown. In this study, the expression of several homing markers and the migratory profile of circulating DC subsets towards intestinal chemo-attractants were evaluated and the effect of biological drugs with different mechanisms of action, such as anti-TNFα or anti-integrin α4β7 (vedolizumab), on this mechanism in healthy controls (HCs) and IBD patients was also assessed. Our results revealed that type 2 conventional DCs (cDC2) express differential homing marker profiles in UC and CD patients compared to HCs. Indeed, integrin β7 was differentially modulated by vedolizumab in CD and UC. Additionally, although CCL2 displayed a chemo-attractant effect over cDC2, while biological therapies did not modulate the expression of the homing markers, we paradoxically found that anti-TNF-treated cDC2 increased their migratory capacity towards CCL2 in HCs and IBD. Our results therefore suggest a key role for cDC2 migration towards the intestinal mucosa in IBD, something that could be explored in order to develop novel diagnostic biomarkers or to unravel new immunomodulatory targets in IBD.

CD38 is a multifunctional molecule that functions both as a transmembrane signaling receptor and as an ectoenzyme with important roles in cell adhesion, calcium regulation and signal transduction. Within the B cell linage, CD38 is expressed in diverse murine B cell subsets, with highest levels in innate B cell subpopulations such as marginal zone (MZ) B cells or B1 cells. In humans, however, CD38 is transiently expressed on early lymphocyte precursors, is lost on mature B cells and is consistently expressed on terminally differentiated plasma cells. In the present work, we have identified two homologues of mammalian CD38 in rainbow trout ( Oncorhynchus mykiss ), designating them as CD38A and CD38B. Although constitutively transcribed throughout different tissues in homeostasis, both CD38A and CD38B mRNA levels were significantly up-regulated in head kidney (HK) in response to a viral infection. In this organ, after the generation of a specific monoclonal antibody (mAb) against CD38A, the presence of CD38A + populations among IgM + B cells and IgM - leukocytes was investigated by flow cytometry. Interestingly, the percentage of IgM + CD38A + B cells increased in response to an in vitro stimulation with inactivated Aeromonas salmonicida . Finally, we demonstrated that HK IgM + CD38A + B cells had an increased IgM secreting capacity than that of cells lacking CD38A on the cell surface, also showing increased transcription levels of genes associated with B cell differentiation. This study strongly suggests a role for CD38 on the B cell differentiation process in teleosts, and provides us with novel tools to discern between B cell subsets in these species., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Martín, Perdiguero, Morel, Soleto, Herranz-Jusdado, Ramón, Abós, Wang, Díaz-Rosales and Tafalla.)

In mammals, Blimp1 (B lymphocyte-induced maturation protein 1) encoded by the prdm1 gene and its homolog Hobit (homolog of Blimp1 in T cells) encoded by znf683 , represent key transcriptional factors that control the development and differentiation of both B and T cells. Despite their essential role in the regulation of acquired immunity, this gene family has been largely unexplored in teleosts to date. Until now, one prdm1 gene has been identified in most teleost species, whereas a znf683 homolog has not yet been reported in any of these species. Focusing our analysis on rainbow trout ( Oncorhynchus mykiss ), an in silico identification and characterization of prdm1 -like genes has been undertaken, confirming that prdm1 and znf683 evolved from a common ancestor gene, acquiring three gene copies after the teleost-specific whole genome duplication event (WGD) and six genes after the salmonid-specific WGD. Additional transcriptional studies to study how each of these genes are regulated in homeostasis, in response to a viral infection or in B cells in different differentiation stages, provide novel insights as to how this gene family evolved and how their encoded products might be implicated in the lymphocyte differentiation process in teleosts., (Copyright © 2020 Perdiguero, Goméz-Esparza, Martín, Bird, Soleto, Morel, Díaz-Rosales and Tafalla.)

As B cells are singularly equipped with a B cell receptor (BCR) and a range of innate receptors, they are able to integrate both antigen-specific and innate signals, with the latter being essential to reach an adequate level of activation. Whether teleost B cells sense pathogens through innate mechanisms has not yet been explored, despite the fact that fish B cells display a wider array of innate receptors than many mammalian B cell subsets. Hence, in the current study, we have investigated the effects of inactivated Aeromonas salmonicida, a Gram negative rainbow trout pathogen, on trout splenic IgM + B cells in vitro in the presence or absence of different inhibitors of Toll-like receptor (TLR) signalling, to establish to what degree innate signals are contributing to the activation of B cells in teleosts. Our results demonstrate that most of the effects that A. salmonicida exerts on trout IgM + B cells are significantly blocked in the presence of inhibitors of MyD88 and TRIF, important nodes in TLR signal pathways. Thus, the data presented demonstrates that, also in teleost, TLR signalling is essential for the activation of IgM + B cells. These results will be useful for the future optimization of novel vaccines and adjuvants.

Interferons (IFNs) orchestrate antiviral responses in jawed vertebrates and can be classified into three types based on different aspects of their genomic organization, structure and receptors through which they signal and function. Generally, type I and type III IFNs include cytokines that directly induce an antiviral response, whereas type II IFNs are well-known for their immunomodulatory role during viral infections. In mammals, type I IFNs have been shown to also regulate many aspects of B cell development and differentiation. Yet, these functions have been only faintly investigated for teleost IFNs. Thus, in the current study, we have examined the effects of a model type I rainbow trout IFN molecule (IFNa) on blood naïve (IgM + IgD + ) B cells, comparing them to those exerted by type II IFN (IFNγ). Our results demonstrate that IFNa increases the survival of naïve rainbow trout B cells, in the absence of lymphoproliferative effects, by rescuing them from spontaneous apoptosis. Additionally, IFNa increased the phagocytic capacity of blood IgM + IgD + B cells and augmented the number of IgM-secreting cells in blood leukocyte cultures. IFNγ, on the other hand, had only minor effects up-regulating IgM secretion, whereas it increased the phagocytic capacity of IgM - cells in the cultures. Finally, given the recent identification of 9 mx genes in rainbow trout, we have also established which of these genes were transcriptionally regulated in blood naïve B cells in response to IFNa. This study points to a previously undescribed role for teleost type I IFNs in the regulation of B cell responses., (Copyright © 2020 Benedicenti, Wang, Morel, Secombes, Soleto, Díaz-Rosales and Tafalla.)

Immunoglobulin D (IgD) is an ancient antibody with dual membrane-bound and fluid-phase antigen receptor functions. The biology of secreted IgD remains elusive. Here, we demonstrate that teleost IgD + IgM - plasmablasts constitute a major lymphocyte population in some mucosal surfaces, including the gut mucosa. Remarkably, secreted IgD binds to gut commensal bacteria, which in turn stimulate IgD gene transcription in gut B cells. Accordingly, secreted IgD from gut as well as gill mucosae, but not the spleen, show a V(D)J gene configuration consistent with microbiota-driven clonal expansion and diversification, including mild somatic hypermutation. By showing that secreted IgD establishes a mutualistic relationship with commensals, our findings suggest that secreted IgD may play an evolutionary conserved role in mucosal homeostasis., (Copyright © 2019 The Author(s). Published by Elsevier Inc. All rights reserved.)

In the absence of class switch recombination and germinal centers, the mechanisms through which B cells from teleost fish mount extrafollicular immunoglobulin M (IgM) memory responses remains mostly unexplored. In this report, we demonstrate that teleost IgM + B cells respond to CD40L, a thymus-dependent activation signal, similarly to mammalian B2 cells. However, when stimulated with different types of antigens, fish IgM + B cells only reach a general activation state in response to antigens cataloged as thymus-independent 1 (TI-1) in mammals, as established through both functional assays and RNA sequencing. Interestingly, fish IgM + B cells remained completely unresponsive to TI-2 antigens, suggesting that the engagement of innate receptors provided by TI-1 antigens is required for the activation of teleost B cells. Finally, a synergy between CD40L and TI-1 antigens was also demonstrated, further supporting that there is no clear dichotomy between thymus-dependent and TI responses in teleost fish., (Copyright © 2019 Granja, Perdiguero, Martín-Martín, Díaz-Rosales, Soleto and Tafalla.)

Although viruses represent a major threat for cultured fish worldwide, the commercialization of vaccines capable of providing effective and long-lasting protection is still lacking for most of these viral diseases. In this situation, the use of supplemented diets could be a suitable strategy to increase the immune status of the fish and reduce the impact of viral pathogens. Among possible immunostimulants that could be included in these functional feeds, some studies have previously shown that certain β-glucans can significantly increase certain immune parameters of fish and reduce the impact of viral diseases. However, the mechanisms through which β-glucans exert their activity have not been fully elucidated yet. In the current study, we have studied the immune response of different tissues to viral haemorrhagic septicaemia virus (VHSV) in rainbow trout fed with a non-supplemented control diet as well as in fish fed a commercial functional aquafeed (Protec™, Skretting) containing β-glucans, vitamin C, vitamin E and zinc. For this, after 30 days of feeding the fish with one of the two diets, they were subsequently infected with VHSV by bath or mock-infected. After 2 or 6 days post-infection, fish were sacrificed and the levels of transcription of different immune genes such as IgM, IgT, IgD, Mx, interferon γ (IFN γ) and perforin studied in different tissues (kidney, gut and gills). Additionally, the levels of natural IgMs in serum were also determined. Our results demonstrate that fish fed the functional diet were capable of mounting an increased IgM, IgT, IgD and Mx transcriptional response to the virus. Additionally, these fish also showed increased levels of natural IgMs in serum. These results reveal a previously undescribed effect of functional diets on fish Ig production and point to Protec™ as an adequate diet to be incorporated in holistic programs aimed at mitigating the effect of viral diseases., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Dendritic cells (DCs) are professional antigen presenting cells located at mucosal surfaces and lymphoid tissues. Their main role is to present antigens to T cells and thus regulate the initiation of the acquired immune response and modulate tolerance mechanisms towards self-antigens. Despite their relevance, not many studies have addressed the identification and characterization of specific DC subsets in teleost fish. Previous studies in our group identified a DC subpopulation co-expressing CD8α and major histocompatibility complex II (MHC II) on the cell surface in rainbow trout (Oncorhynchus mykiss) skin and gills. A complete functional and phenotypical characterization of these cell subsets was then undertaken, unequivocally recognizing them as DCs (CD8 + DCs). In the current study, we report the identification of a homologous population in rainbow trout intestinal lamina propria (LP). We have studied the main features of these intestinal CD8 + DCs, comparing them to those of CD8 + DCs from another mucosal tissue (gills). Interestingly, intestinal CD8 + DCs exhibited significant phenotypical and functional differences when compared to gill CD8 + DCs, suggesting that the location of DCs strongly conditions their activation state. These results will contribute to further expand our knowledge on how intestinal immune responses are regulated in fish, helping us to rationally design oral vaccines in the future., (Copyright © 2019. Published by Elsevier Ltd.)

Tumor necrosis factor ligand superfamily members such as B cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL) have been identified in mammals as key regulators of B cell homeostasis and activation. However, the immune functions of APRIL are not as well defined as those of BAFF. Furthermore, while BAFF is present in all vertebrates, APRIL is missing in some animal groups, suggesting that BAFF has compensated the functions of APRIL in these species. In this context, we thought of great interest to explore the effects of APRIL on teleost B cells, given that APRIL appears for the first time in evolution in bony fish. Thus, in this study, we have performed an extensive analysis of the effect of APRIL on B cells using rainbow trout ( Oncorhynchus mykiss ) as a model species. Our results demonstrate that APRIL induces a specific proliferation of IgM + B cells by itself and increases IgM secretion without promoting a terminal differentiation to plasma cells. APRIL also increased the levels of surface MHC II and augmented the capacity of these cells to process antigen, effects that were exclusively exerted on IgM + B cells. Although our results point to a highly conserved role of APRIL on B cell homeostasis and activation throughout evolution, some specific differential effects have been observed in fish in comparison to the effects of APRIL previously described in mammals. Finally, the effects that APRIL induces on rainbow trout IgM + B cells described in this paper have been compared with those previously reported in response to BAFF.

Dendritic cells (DCs) are highly specialized antigen-presenting cells that bridge innate and adaptive immune responses in vertebrates, being key modulators in the initiation of specific responses. Although teleost fish present the main elements of a fully developed adaptive immune system, not many studies have focused on identifying specific DC subsets in teleost species. Previous work from our group identified in rainbow trout ( Oncorhynchus mykiss ) skin a DC subpopulation co-expressing CD8α and major histocompatibility complex II β on the cell surface. Interestingly, these CD8 + DCs expressed common unique markers of mammalian cross-presenting DCs, a DC subset with an important role in antigen presentation and activation of CD8 + T cytotoxic lymphocytes. In this study, we have identified a similar DC subset in rainbow trout gills that also transcribes molecules uniquely expressed on diverse mammalian cross-presenting DC populations such as CD8, CD103, CD141, Batf3, IFN regulatory protein 8, and toll-like receptor 3. Hence, we have undertaken a broad phenotypic and functional characterization of this new DC subset that includes the confirmation of novel capacities for DCs in teleost, such an IgM-binding capacity and responsiveness to CD40 ligand. Furthermore, our results show that in gills, this DC subset shows some different phenotypic and functional characteristics when compared with their homologs in the skin, suggesting an adaptation of the cells to different mucosal tissues or different maturation status depending on their location. Our findings contribute to increase our knowledge on fish cross-presenting DCs, an important cell population to take into account for the future design of mucosal vaccination strategies.

IgM + B cells have been recently demonstrated to be key regulators of peritoneal inflammation in teleost, as a large number of them occupy the peritoneal cavity after 48 h of antigenic stimulation. Despite this, the number of studies addressing the mechanism through which this cell population expands and differentiates in response to stimuli has been scarcely addressed. Because the BAFF/APRIL axis is known to play a major role in B cell survival and differentiation in mammals, we hypothesized that it could be affected in the peritoneal cavity in response to an inflammatory stimulus. To verify this hypothesis, we studied how BAFF, APRIL and the fish-specific related cytokine BALM as well as their putative receptors are regulated in rainbow trout after intraperitoneal (i.p.) injection of viral hemorrhagic septicemia virus (VHSV). When the transcriptional analysis was performed in total cells from the peritoneum, we observed that VHSV provoked an up-regulation of both BAFF and BAFF receptor (BAFF-R) mRNA levels. However, when we examined how isolated peritoneal IgM + B cells were transcriptionally affected by VHSV i.p. injection, we found that APRIL, BALM and the transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) were also up-regulated in response to the virus. IgM - cells, on the other hand, only up-regulated BALM transcription in response to VHSV. Finally, to gain further insight on the role that these cytokines play in the peritoneum, we have studied their effect on the survival of peritoneal IgM + B cells. This work demonstrates a key role for the BAFF/APRIL axis in the peritoneal inflammatory response and contributes to further understanding how IgM + B cells are regulated at this specific peripheral site., (Copyright © 2017 Elsevier Ltd. All rights reserved.)