The most remarkable feature of the immune response is its ability to generate cells and antibodies specifically reactive with an enormous variety of foreign antigens while normally avoiding the production of autoreactive elements. An accumulating body of data demonstrates that a distinct lineage of B cells (1), the Ly-1 + B cell subset, does not display this feature, but rather is associated with the production ofautoreactive antibodies (2). We are unaware of autoimmune pathology directly demonstrated to be the result of an Ly-1 + B cell-Ig product; however, Ly-1+ B cell frequency is elevated in some autoimmune strains of mice (3). In addition to their association with autoreactive antibody, Ly-1 + B cells have been shown to have unique growth potential (1, 4); to produce most of the serum IgM found in normal, unimmunized animals (4); and the Ig repertoire of the subset may be restricted (4, 5). These findings imply that this subset has a role in normal physiology, and perhaps autoimmune pathology. A B cell subset has been identified in humans that bears the homologous marker Leu-1/T1 (CD5) (6). A similar functional role for CD5+ B cells in man has been proposed (7); human CD5+ B cells have been shown to produce autoreactive antibody (8, 9) and to be present at elevated frequency in rheumatoid arthritis (10).Normal mice produce antibodies that are reactive with protease-treated autologous erythrocytes (BrMRBC) 1 (11) ; cells producing such antibodies are present in unimmunized mice at very high frequencies, as first demonstrated by Cunningham (12) . We and others (13)(14)(15) have shown that at least some anti-BrMRBC plaqueforming cells (PFC) recognize the polar headgroup of the common membrane phospholipid, phosphatidyl choline (PtC). Most splenic anti-BrMRBC PFC have been

Most, if not all, autoantibodies specific for bromelain-treated mouse erythrocytes recognize the common membrane phospholipid, phosphatidyl choline (PtC). Anti-PtC antibodies are produced by 5%-15% of CD5+ Ly-1 B cells of normal unimmunized mice, but not by detectable numbers of conventional CD5- B cells. At 1 week of age PtC-specific B cells are undetectable but then increase dramatically over the next 3 to 4 weeks to reach adult numbers. We report here that PtC-specific Ly-1 B cells in B10.H-2aH-4bp/Wts mice predominantly express either of two heavy and kappa chain variable (V) region gene combinations. In addition, the sequence and length of DH genes are conserved among cells expressing the same V gene combination, and the V kappa-J kappa junctions of one group involve unusual splice sites. Preferential V gene rearrangement models are insufficient to explain the DH and V kappa-J kappa junctional sequences or the delayed appearance of this specificity, and so they cannot solely account for the high frequency of PtC-specific cells. These characteristics are more consistent with antigen selection. We therefore attribute the frequent use of the two V region gene combinations to selection for cells that express them and conclude that the expressed V gene repertoire of Ly-1 B cells in adult mice is influenced by antigen selection. Apparently, there is no selection for mutant anti-PtC antibodies of higher affinity during the formation of the Ly-1 B repertoire because the V region genes expressed by PtC-specific cells are unmutated. Our findings are consistent with an important, germ line-encoded function for the immunoglobulin products of these gene combinations.

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A minor population of murine B iymphocytes express the cell surface antigen Ly-1 (1), and a similarly small proportion of human B cells carry the homologous antigen Leu-1/T1 (2). In normal mice, ~2-5% of splenic B cells bear Ly-1, but these include virtually all of the cells that spontaneously secrete antibody hemolytic for SRBC or for autologous erythrocytes treated with the proteolytic enzyme bromelain (BrMRBC). 1 In NZB mice, which develop congenital autoimmune disease, the frequency of Ly-1 + B cells is elevated (3), and they are reportedly responsible for the production of all the anti-DNA antibodies associated with the disease (4).We have previously described (5) 27 independently derived murine B cell lymphomas in the CH series. At least 70% of these tumors bear Ly-1, their Ig are idiotypicaily related one to another, and several recognize SRBC and Br-MRBC via their surface Ig (5). The spleens of normal syngeneic mice contain Ly-1 + B cells that produce antibody hemolytic for SRBC and BrMRBC. About half of these cells are idiotypically crossreactive with the erythrocyte-reactive CH lymphomas (6). This implies that the CH lymphomas represent neoplastic analogs of the Ly-1 + normal B cells.During an experiment using antibody-conjugated, dye-loaded liposomes as a second-step immunofluorescence reagent, we noted that CH 12 tumor cells bound them independently of the addition of first-step antibody. Binding was not a function of the specificity of the antibody conjugated to the liposomes, but occurred with all liposomes of similar phospholipid composition. Given that the liposomes are constructed with phospholipids similar to those present in erythrocyte membranes and that some erythrocytes bind to members of the CH series by way of surface IgM (sIgM), we decided to explore the possibility that the particular epitope recognized by CH 12 Ig was included as a component of the fluorescent dye-loaded liposomes.