Inhibition by platelets of in vitro blastogenic responses of canine PBMC by a PAF-dependent mechanism

It was demonstrated that platelets can inhibit in vitro blastogenic responses by a platelet activating factor (PAF)-dependent mechanism. A procedure for the isolation of virtually platelet-free canine peripheral blood mononuclear cells (PBMC), using Ficoll density gradient centrifugation followed by Percoll density centrifugation, was developed to investigate the mechanism by which platelets inhibit the in vitro blastogenic response of PBMC. It was shown that PBMC purified on Ficoll gradients alone are contaminated with platelets and proliferate weakly compared with platelet-free PBMC purified on an additional Percoll gradient. Addition of platelets to PBMC cultures in the presence of PAF receptor antagonist resulted in a proliferative response similar in intensity to that of platelet-free PBMC cultures, whereas the addition of platelets to PBMC cultures in the absence of PAF receptor antagonist resulted in marked inhibition of the mitogen-induced proliferative response. Therefore, PAF is likely to be involved in the inhibition of in vitro proliferative responses of platelet-contaminated canine PBMC.