Central role of CD4+ T cells in avian immune response.
Chicken alpha beta T cells express either CD4 or CD8 accessory molecules, whereas most of the gamma delta T cells do not. The functional significance of the alpha beta T cells is relatively well understood. The CD4+ alpha beta T cells function as coordinators of the immune response, and CD8+ alpha beta T cells are the effector cells in cytotoxic responses, killing infected target cells. In comparison, the role of gamma delta T cells is so far poorly known. In chicken, the gamma delta T cells comprise a large lymphocyte subset. They can be induced to proliferate by various stimuli, but the proliferative response is dependent on CD4+ alpha beta T cells. The CD4+ T cells are also essential for the generation of antibody responses by providing help for the B cells and can influence cytotoxic responses as well. Thus, the CD4+ alpha beta T cells have a central role in the avian immune system, and their activation is a prerequisite for responses by other types of cells, including gamma delta T cells.