Interleukin 9-induced in vivo expansion of the B-1 lymphocyte population

The activity of interleukin (IL)-9 on B cells was analyzed in vivo using tr ansgenic mice that constitutively express this cytokine. These mice show an increase in both baseline and antigen-specific immunoglobulin concentratio ns for all isotypes tested. Analysis of B cell populations showed a specifi c expansion of Mac-1(+) B-1 cells in the peritoneal and pleuropericardial c avities, and in the blood of IL-9 transgenic mice. In normal mice, the IL-9 receptor was found to be expressed by CD5(+) as well as CD5(-) B-1 cells, and repeated injections of IL-9 resulted in accumulation of B-1 cells in th e peritoneal cavity, as observed in transgenic animals. Unlike other mouse models, such as IL-5 transgenic mice, in which expansion of the B-1 populat ion is associated with high levels of autoantibodies, IL-9 did not stimulat e the production of autoantibodies in vivo, and most of the expanded cells were found to belong to the B-1b subset (IgM(+)Mac-1(+)CD5(-)). In addition , we found that these IL-9-expanded B-1b cells do not share the well-docume nted antibromelain-treated red blood cell specificity of CD5(+) B-1a cells. The increase of antigen-specific antibody concentration in immunized mice suggests that these B-1 cells are directly or indirectly involved in antibo dy responses in IL-9 transgenic mice.