Spontaneously Ig-secreting B-1 cells violate the accepted paradigm for expression of differentiation-associated transcription factors.

B-1 cells spontaneously secrete natural Ig that acts as a primary line of defense against infection. A major shortfall in our understanding of this key process centers on the molecular mechanisms regulating natural Ab secretion by B-1 cells. Herein, we demonstrate that secreting B-1 cells use some aspects of the recently recognized plasmacytic differentiation program but deviate from it in important ways. Specifically, we show that key repressors of the plasmacytic program, B cell leukemia/lymphoma-6 and paired box gene 5, are reduced in spontaneously secreting B-1 B cells, as in stimulated differentiated B-2 cells. Surprisingly, we find that key promoters of the plasmacytic program, B lymphocyte inducer of maturation program 1 and X-box binding protein 1, are not up-regulated in secreting B-1 cells, in contrast to secreting B-2 cells. These data demonstrate that B-1 cells operate under a differentiation program that is unique and differs from the paradigm associated with Ig-secreting B-2 cells.