B cells inhibit induction of T cell-dependent tumor immunity

Cytotoxic T lymphocyte (CTL) mediated tumor immunity against major histocompatibility antigen (MHC) class I-positive but class Ii-negative tumors often requires help from CD4(+) T cells. These CD4 cells are activated by MHC class Ii-positive cells that present tumor derived antigens(1,2). Considering that different antigen presenting cells, such as B cells, macrophages and dendritic cells compete for antigen and influence the outcome of an immune response(3-13), we analyzed tumor immunity in B cell-deficient mice(14). These mice appear normal with regard to T cell immunity and tolerance to some pure foreign antigens(15-19), We show here that the low immunogenicity of tumors is caused by B cells whose presence in the priming phase results in disabled CD4(+) T cell help for CTL mediated tumor immunity. Instead, in the presence of B cells, a non-protective humoral immune response is induced. Our results may explain the enigmatic observation that tumor-reactive antibodies occur frequently in cancer patients(20).