Primate B-1 cells generate antigen-specific B cell responses to T cell-independent type 2 antigens

Ab responses to T cell-independent type 2 (TI-2) Ags, such as bacterial capsular polysaccharides, are critical for host defense. In mice, B-1b cells expressing a CD11b(+)FSC(hi)CD21(lo/-)CD19(hi) phenotype play a key role in producing Abs against TI-2 Ags. In primates, a distinct IgM(+)CD27(+) "memory" B cell population is thought to generate TI-2 Ab responses, and evidence for a B-1b-like cell population participating in these responses is lacking. In this article, we demonstrate that nonhuman primates (NHPs; African green monkeys and cynomolgus macaques) harbor serosal B cells expressing a CD11b(+)FSC(hi)CD21(lo/-)CD80(+/-)CD19(hi) phenotype, constitutively active Stat3, and increased reactivity with phosphorylcholine, similar to murine peritoneal B-1a and B-1b cell populations. Like what is observed for murine B-1b cells, NHP CD11b(+)FSC(hi)CD21(lo/-)CD19(hi) B cells dominate the Ag-specific B cell response and Ab production against the TI-2 Ag trinitrophenyl-Ficoll. Although Ag-specific IgM(+) B cells expressing CD27 were not detected prior to immunization, Ag-specific CD11b(+)CD19(hi) B cells expressed and maintained an IgM(+)IgD(lo)CD27(+)CD80(+) phenotype following immunization. Thus, the murine and NHP B cell populations responding to trinitrophenyl-Ficoll are highly similar, with the main exception being that Ag-specific NHP B-1-like cells express CD27 following TI-2 Ag encounter. Therefore, murine B-1b and primate IgM(+)CD27(+) "memory" B cell subsets proposed to produce TI-2 Ab responses may be highly related, if not identical. Overall, these data not only support that B-1-like cells are present in NHPs but also provide evidence that these cells perform the same functions attributed to murine B-1b cells.