The LY-1B Cell Lineage

A small subpopulation of normal murine splenic B cells carrying all of the classic B cells markers (IgM, IgD, Ia, and ThB) also carries Ly-1, one of the major T cell surface molecules. This "Ly-1 B" subpopulation (identified and characterized by multiparameter FACS analyses) consists of relatively large, high IgM/low-IgD/low-Ly-1 lymphocytes that represent approximately 2% of the spleen cells in normal animals and, generally, 5-10% of spleen cells in NZB mice. Ly-1 B are clearly detectable in all normal mouse strains tested as well as NZB, CBA/N, other X-id mice and nude (nu/nu) mice. They are found primarily in the spleen; are either absent or very poorly represented in lymph node, bone marrow, and thymus; appear early during ontogeny, and comprise about a third of the small number of lymphocytes present in 5-d-old mice. NZB and (NZB x NZW)F1 mice have more Ly-1 B than all other strains and, furthermore, have a unique Ly-1 B population that secretes IgM when cultured under usual conditions in the absence of added antigen. The IgM secretion by these Ly-1 B cells accounts for the previously reported "spontaneous" IgM secretion by NZB spleen cells in culture. Studies with FACS-sorted cells show that the presence of Ly-1 on these IgM-secreting cells distinguishes them from the (Ly-1 negative) IgM-secreting "direct" plaque-forming cells generated in NZB mice after stimulation with sheep erythrocytes.

Data from previous multiparameter fluorescence-activated cell sorter (FACS) analysis and sorting studies define a subset of murine B cells that expresses the Ly-1 surface determinant in conjunction with IgM, IgD, Ia, and other typical B cell markers. These Ly-1 B cells are physically and functionally distinct. They express more IgM and less IgD than most other B cells; they are not normally found in lymph node or bone marrow; they are always present at low frequencies (1-5%) in normal spleens, and, as we show here, they comprise about half of the B cells (10-20% of total cells) recovered from the peritoneal cavity in normal mice. Furthermore, most of the commonly studied IgM autoantibodies in normal and autoimmune mice are produced by these Ly-1 B cells, even though they seldom produce antibodies to exogenous antigens such as trinitrophenyl-Ficoll or trinitrophenyl-keyhole limpet hemocyanin. Cell transfer studies presented here demonstrate that the progenitors of Ly-1 B cells are different from the progenitors of the predominant B cell populations in spleen and lymph node. In these studies, we used FACS analysis and functional assays to characterize donor-derived (allotype-marked) B cells present in lethally irradiated recipients 1-2 mo after transfer. Surprisingly, adult bone marrow cells typically used to reconstitute B cells in irradiated recipients selectively failed to reconstitute the Ly-1 B subset. Liver, spleen, and bone marrow cells from young mice, in contrast, reconstituted all B cells (including Ly-1 B), and peritoneal "washout" cells (PerC) from adult mice uniquely reconstituted Ly-1 B. Bone marrow did not block Ly- 1 B development, since PerC and newborn liver still gave rise to Ly-1 B when jointly transferred with marrow. These findings tentatively assign Ly-1 B to a distinct developmental lineage originating from progenitors that inhabit the same locations as other B cell progenitors in young animals, but move to unique location(s) in adults.