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Funding This work was supported by the European Research Council (ERC Consolidator Grant 2016 725061 TEMUBLYM) and by projects AGL2014-54456-JIN (Spanish Ministry of Economy and Competitiveness, MINECO) and AGL2017-85494-C2-1-R (Spanish Ministry of Science, Innovation and Universities)., 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, Depto. de Genética, Fisiología y Microbiología, Fac. de Ciencias Biológicas, TRUE, pub

In the original article, there was a mistake in Figure 1 as published. Although the numbers were correct, all dot plots in Figure 1A were the same. The corrected Figure 1 appears below. The authors apologize for this error and state that this does not change the scientific conclusions of the article in any way. The original article has been updated. © 2021 Benedicenti, Wang, Morel, Secombes, Soleto, Dı́az-Rosales and Tafalla. This corrects the article DOI: 10.3389/fimmu.2020.01494.

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.

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.

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.

Funding This work was supported by the European Research Council (ERC Consolidator Grant 2016 725061 TEMUBLYM), by the Spanish Ministry of Science, Innovation and Universities (project AGL2017-85494-C2-1-R) and by the Comunidad de Madrid (grant 2016-T1/BIO-1672)., 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., Depto. de Genética, Fisiología y Microbiología, Fac. de Ciencias Biológicas, TRUE, pub

Acknowledgments Lucia González and Beatriz Abós are greatly acknowledged for technical assistance. This work was supported by the European Research Council (ERC Consolidator Grant 725061), the Spanish Ministry of Science, Innovation, and Universities (project AGL2017-85494-C2-1-R), and the Comunidad de Madrid (grant 2016-T1/BIO-1672)., 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., Depto. de Genética, Fisiología y Microbiología, Fac. de Ciencias Biológicas, TRUE, pub

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 com- pensated 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 differenti- ation 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.

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.