Depression of lymphocyte reactivity by granulocytes in equine whole blood culture

Tritiated thymidine uptake in response to Concanavalin A was recorded as stimulated counts per minute (SCPM) in equine whole blood cultured with an optimum concentration of 3 X 10(4) mononuclear cells/culture. A significant negative correlation was found between log10 SCPM and granulocyte level in culture, in a group of 27 adult horses (r = -0.745, P less than 0.001). Addition of isolated autologous granulocytes to such cultures resulted in a reduction of log10 SCPM of a magnitude similar to that predicted by the gradient of the log10 SCPM/granulocyte level regression line in the group of adult horses. It is concluded that the presence of granulocytes in culture directly depresses lymphocyte reactivity, and that a large part of the variability of SCPM between horses is due to variation of granulocyte levels.