AimsIn the present study we have investigated the comparative switching propensity of murine peritoneal and splenic B cell subpopulations to IgA in presence of retinoic acid (RA) and TGF-β.Methods and ResultsTo study the influence of RA and TGF-β on switching of B cell subpopulations to IgA, peritoneal (B1a, B1b and B2 cells) and splenic (B1a, marginal zone, and B2) B cells from normal BALB/c mice were FACS purified, cultured for 4 days in presence of RA and TGF-β and the number of IgA producing cells was determined by ELISPOT assay or FACS analysis. In presence of TGF-β, peritoneal B1b cells switched to IgA more potently than other peritoneal B cell subpopulations. When TGF-β was combined with retinoic acid (RA), switching to IgA was even more pronounced. Under these conditions, “innate” B cells like peritoneal and splenic B1 cells and MZ B cells produced IgA more readily than B2 cells. Additionally, high frequency of nucleotide exchanges indicating somatic hypermutation in VH regions was observed. Besides IgA induction, RA treatment of sorted PEC and splenic B cells led to expression of gut homing molecules - α4β7 and CCR9. Intraperitoneal transfer of RA-treated B1 cells into Rag1-/- recipients resulted in IgA in serum and gut lavage, most efficiently amongst B1b cell recipients.ConclusionPresent study demonstrates the differential and synergistic effect of RA and TGF-β on switching of different B cell subpopulations to IgA and establishes the prominence of peritoneal B1b cells in switching to IgA under the influence of these two factors. Our study extends our knowledge about the existing differences among B cell subpopulations with regards to IgA production and indicates towards their differential contribution to gut associated humoral immunity.

IntroductionIgA is the major class of Ab present in mucosal tissues of mammals. IgA constitutes a key defense mechanism against invasion by inhaled or ingested pathogens. IgA is also found at significant concentrations in the serum of many species, where it mediates the elimination of pathogens that have breached the mucosa [1].Among the various MALTs, IgA-producing cells are present in highest numbers in GALTs constituted by Peyer's patches (PPs), isolated lymphoid follicles (ILFs), and solitary intestinal lymphoid Correspondence: Dr. Bishnudeo Roy e-mail: Bishnudeo.Roy@helmholtz-hzi.de tissue (SILT) [2,3]. B cells, present in the B-cell follicles of such organized structures, form GCs upon Ag encounter. Under the influence of various cellular and molecular factors -T cells and cytokines, for instance -these B cells are thought to switch from IgM to IgA [4].Multiple pathways of IgA induction have been reported. The T-cell-dependent pathway of IgA induction is well known and, it is clear that IgA can also be induced in T-cell-independent (TI) manner (e.g. in T-cell-deficient mice, CD40 −/− mice, CD28 −/− mice, etc.) [5][6][7]. However, the precise contribution of T-cell-dependent and TI pathways to IgA production is not known.An important contribution of B-1 cells to TI responses has been suggested [6,8,9]. Peritoneal cavity (PEC) B-1 cells have also been claimed to contribute significantly to IgA-producing plasma cell (PC) Eur. J. Immunol. 2013Immunol. . 43: 2023Immunol. -2032. Importantly, according to phenotype, origin, and function, PEC B-1 cells can be divided into B-1a and B-1b subpopulations [12,13]. Phenotypically, B-1a cells are characterized as B220 lo CD19 hi IgM hi IgD lo CD43 + Mac-1 + CD5 int . B-1b cells share all the aforementioned markers with B-1a cells except CD5. With respect to differential contribution of these two B-1-cell subtypes to IgA production, recently, we could show that most of the IgA-secreting cells in the PEC of unmanipulated mice belonged to B-1b-cell subpopulation [14]. Additionally, IgA-derived VH regions from B-1b cells also contained frequent single nucleotide exchanges indicative of somatic hypermutation (SHM) [14]. Thus, a contribution of PEC B-1b cells to IgA production and hence to gut-associated immunity is quite likely. However, in spite of evidential suggestions for the contribution of B-1 cells to the intestinal PC pool, their contribution under unmanipulated conditions remains uncertain. This is partly due to the lack of appropriate markers to distinguish PCs according to their origin. In this context, a sequence-based approach to address this question should become very useful.In the present work, to investigate the participation of PEC B-1 cells in the gut-associated IgA production we have made use of the transgenic mouse model known as the L2 mouse line [15]. Mice of this line are characterized by the expression of a transgenic λ light chain obtained from the plasmacytoma MOPC315 (λ2 315 ). PEC of these mice contains almost exclusively CD5 + B-1a cells, whi...

We employed the B-Indu-Rag1 model in which the coding exon of recombination-activating gene 1 (Rag1) is inactivated by inversion. It is flanked by inverted loxP sites. Accordingly, B cell development is stopped at the pro/pre B-I cell precursor stage. A B cell-specific Cre recombinase fused to a mutated estrogen receptor allows the induction of RAG1 function and B cell development by application of Tamoxifen. Since Rag1 function is recovered in a non-self-renewing precursor cell, only single waves of development can be induced. Using this system, we could determine that B cells minimally require 5 days to undergo development from pro/preB-I cells to the large and 6 days to the small preB-II cell stage. First immature transitional (T) 1 and T2 B cells could be detected in the bone marrow at day 6 and day 7, respectively, while their appearance in the spleen took one additional day. We also tested a contribution of adult bone marrow to the pool of B-1 cells. Sublethally irradiated syngeneic WT mice were adoptively transferred with bone marrow of B-Indu-Rag1 mice and B cell development was induced after 6 weeks. A significant portion of donor derived B-1 cells could be detected in such adult mice. Finally, early VH gene usage was tested after induction of B cell development. During the earliest time points the VH genes proximal to D/J were found to be predominantly rearranged. At later time points, the large family of the most distal VH prevailed.