The results show that CD1d expression and iNKT cells are dispensable for the development, accumulation, or IL10 competence of CD5+ B cells in mice prone to benign or leukemic CLL-like B cell expansion, but reveal a novel role for iNKT cells in supporting B cell progression through the transitional stage of development in these animals. Here, IL10 is linked to B-cell chronic lymphocytic leukemia.