5and = 5, per group). and establishment of continual FtL-specific B-1a storage occur in the lack of adjuvants readily, IL-7, T cells, or germinal middle support. Nevertheless, in another proclaimed departure through the mechanisms managing B-2 storage replies, rechallenge with FtL within an inflammatory context is required to induce B-1a secondary antibody responses. These findings introduce previously unexplored vaccination strategies for pathogens that target the B-1a repertoire. live-vaccine strain (LVS) readily induces splenic FtL-specific B-1a to Alexidine dihydrochloride produce T-independent antigen-specific IgM and IgG (IgM IgG) primary antibody responses, to develop long-term antigen-specific memory, and to produce secondary antibody responses when appropriately rechallenged with the antigen. Strikingly, although the B-1a memory responses that we identify Alexidine dihydrochloride share many of the properties of B-2 memory responses, they nonetheless differ in key ways that make the B-1a responses more suitable for the functional niche they occupy. B-1 lymphocytes represent 1C5% of total B cells in adult mice. They are the principal B cells in peritoneal (PerC) and pleural cavities, are present at low but detectable frequencies in spleen and intestine, and are very rare in bone marrow (BM) and lymph nodes (1, 3). B-1a, which express low levels of CD5, predominate among PerC B-1, but B-1b, which do not express CD5, are present at much lower frequencies in the PerC (2, 3). Functionally, B-1a are well known to produce natural antibodies (2, 5C8) and to up-regulate the antibody production in response to Toll-like receptor (TLR) stimulation (9C11). Consistent with this function, our recent studies show that stimulation with LPS, a TLR4 agonist, nonspecifically induces PerC B-1a to migrate to spleen, where they join with resident splenic B-1a to augment polycolonal antibody production (10). In addition, intranasal influenza infection has been shown to induce B-1a migration, in this case to respiratory tract lymphoid organs where, without undergoing clonal TNFRSF16 expansion, the migrants produce IgM that includes virus-neutralizing natural IgM antibodies (12). These findings fuel the prevailing view Alexidine dihydrochloride that B-1a do not mount antigen-induced antibody responses (13). However, B-1a are clearly known to produce specific antibody responses to certain antigens, including phosphorylcholine (14C16) and 1,3 dextran (17, 18). Most recently, foreshadowing studies Alexidine dihydrochloride presented here, we have shown that immunization with FtL, an atypical LPS isolated from LVS, induces B-1a with FtL-binding IgM receptors to appear in spleen and to produce anti-FtL IgM primary antibody responses that protect against lethal LVS challenge (19, 20). Consistent with B-1a mediating this protection, FtL priming does not similarly protect Bruton’s tyrosine kinase (Btk)-mutant (infection such that transferring sorted PerC B-1b from infected mice intravenously to Rag1?/? mice confers long-term protection (24). Confirming that B-1b rather than B-1a mediate this protection, immunization also protects the Btk-mutant mice mentioned above, which lack B-1a (25). Thus, B-1a and B-1b have distinct repertoires and response properties. Studies here and in a companion article (4) together show that the B-1aCmediated protection that FtL priming induces against lethal LVS challenge (19) is accompanied by induction of anti-FtL B-1a primary responses and, importantly, by induction of anti-FtL B-1a memory cells that persist indefinitely in PerC (but not elsewhere) and remain ready to respond to FtL rechallenge under appropriate conditions. Activation-induced cytidine deaminase (AID)-dependent isotype switching occurs during development of a proportion of these FtL-specfic B-1a memory cells. However, unlike B-2 memory, B-1a memory cells develop in the absence of T-cell or germinal center (GC).