A functional characterization of macrophage alterations in casein-treated B6C3F1 mice

We have previously reported that subcutaneous injection of casein, a potent inducer of the immunomodulatory acute phases reactant, serum amyloid A (SAA) protein, produces a marked suppression of humoral responses that require macrophage accessory cell cooperativity in the B6C3F1 mouse. The objective of these studies was to further characterize the immunological changes produced by casein treatment. It was observed that the inhibition of the sRBC IgM AFC response which accompanies casein treatment is dose related to the amount of casein introduced subcutaneously to the mouse. These studies, as well as those previously reported by several laboratories including our own have demonstrated that spleen cells isolated from casein-treated mice also exhibit markedly suppressed humoral responses in vitro. However, casein added directly to naive spleen cell cultures at concentrations significantly higher than those which would be found in the lymphoid tissues of the intact animal have no direct inhibitory effect on the sRBC IgM AFC response, suggesting that casein alone does not exert a direct immunosuppressive effect. Kinetics of recovery studies indicate that the casein-induced immunosuppression is readily reversible. Humoral responses are fully recovered within 3 days, once subcutaneous injections of casein are terminated. In vitro measurements of IL-1 secretion following stimulation of splenic macrophages, isolated from casein treated mice, with lipopolysaccharide indicate no significant effect on the capacity of these cells to produce this cytokine. Direct addition of recombinant IL-1 or interferon-gamma to spleen cell cultures isolated from casein-treated mice also was found to be incapable of reversing the inhibited IgM AFC response. Taken together, these studies strongly suggest that the accessory cell dysfunction associated with macrophages from casein-treated mice is not due to the inability of these cells to secrete IL-1 and indicate that the dysfunction cannot be reversed by IL-1 or interferon-gamma. Casein treatment was also found to markedly inhibit DTH, a cell-mediated immune response requiring macrophage accessory cell function. interestingly, the DTH responses were only affected by casein when it was administered post-sensitization with antigen (sRBC) but prior to antigen challenge. When casein was administered prior to sensitization with antigen, which is analogous to the treatment schedule that was found to suppress the sRBC antibody response, no effect was observed on DTH.