Fluorescence-activated cell sorter (FACS) analysis of B-lymphocyte surface isotype expression, and limiting dilution B-lymphocyte cloning techniques, have been used to establish characteristics of B lymphocytes from New Zealand Black (NZB) mice which might contribute to the predisposition of this strain to autoimmune disease. The NZB mice and two other strains (BALB/c and CBA) used were either specific pathogen free (SPF) or germ free (GF). The NZB B lymphocytes differed from the normal strains in the following respects: they showed considerably higher spontaneous conversion into antibody-forming cell clones in the absence of antigen or mitogen; substantially higher cloning efficiency at optimal antigen or mitogen concentration; a slightly lower antigen concentration optimum; and an abnormally high proportion of large cells among the s-IgM+, s-IgD- lymphocyte subset. All these differences argue for a heightened excitability of the NZB B cell to triggering stimuli. Although there was a small-to-moderate increase in the proportion of s-IgM+, s-IgD- b cells, the differences in μ:δ ratio were less than those previously reported. Finally, only a minor and barely significant resistance to tolerance induction in vitro was observed. The results suggest that the heightened B-cell excitability is only one factor in the etiology of autoimmune disease.
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