B-1 plasma cells require non-cognate CD4 T cell help to generate a unique repertoire of natural IgM

Evolutionarily conserved, "natural" (n)IgM is broadly reactive to both self and foreign antigens. Its selective deficiency leads to increases in autoimmune diseases and infections. In mice, nIgM is secreted independent of microbial exposure to bone marrow (BM) and spleen B-1 cell-derived plasma cells (B-1PC), generating the majority of nIgM, or by B-1 cells that remain non-terminally differentiated (B-1(sec)). Thus, it has been assumed that the nIgM repertoire is broadly reflective of the repertoire of body cavity B-1 cells. Studies here reveal, however, that B-1PC generate a distinct, oligoclonal nIgM repertoire, characterized by short CDR3 variable immunoglobulin heavy chain regions, 7-8 amino acids in length, some public, many arising from convergent rearrangements, while specificities previously associated with nIgM were generated by a population of IgM-secreting B-1 (B-1(sec)). BM, but not spleen B-1PC, or B-1(sec) also required the presence of TCR alpha beta CD4 T cells for their development from fetal precursors. Together, the studies identify important previously unknown characteristics of the nIgM pool. Natural (n)IgM is critical for immune homeostasis and host defense. Smith et al. show that BM nIgM-secreting B-1 plasma cells require TCR alpha beta(+) CD4 T cell help. Their repertoire lacks known B-1 cell specificities but instead contains public clones with oligoclonal, short Igh-CDR3 motifs.