Ly‐1 B (B‐1) cells are the main source of B cell‐derived interleukin 10

Abstract We have previously reported (O'Garra, A. et al., Int. Immunol. 1990. 2: 821) that murine B lymphomas and purified normal peritoneal B cells produce interleukin (IL) 10. We now show that this production of IL 10 B cells correlates with the presence of Ly‐1 (B‐1) B cells, in both normal and diseased mice. Using a semi‐quantitative modification of the polymerase chain reaction, we show that IL 10 expression is detectable in peritoneal B cells but only becomes apparent in splenic B cells of aged mice of which a high proportion are Ly‐1 + . Furthermore, the expression of IL 10 is constitutive in splenic B cells from mice carrying the Ly‐1 + BCL1 lymphoma. Since IL 10 is a potent regulator of in vitro immune function, its production by Ly‐1 lineage B (B‐1) cells raises the possibility that this subset of B cells may regulate their own development and/or the function of other immunocompetent cells.