IL-4-secreting CD4+ T cells are crucial to the development of CD8+ T-cell responses against malaria liver stages

CD4(+) T cells are crucial to the development of CD8(+) T cell responses against hepatocytes infected with malaria parasites. In the absence of CD4(+) T cells, CD8(+) T cells initiate a seemingly normal differentiation and proliferation during the first few days after immunization. However, this response fails to develop further and is reduced by more than 90%, compared to that observed in the presence of CD4(+) T cells. We report here that interleukin-4 (IL-4) secreted by CD4(+) T cells is essential to the full development of this CD8(+) T cell response. This is the first demonstration that IL-4 is a mediator of CD4/CD8 cross-talk leading to the development of immunity against an infectious pathogen.