Purification and Partial Characterization of a 14 kDa Protein Enhancing Mitogen-Stimulated Proliferation of Human Peripheral Blood Mononuclear Cells.

: Cytotoxicity of plasma free platelets or peripheral blood mononuclear cells (PBMCs) upon stimulation with Ca-ionophore A23187 at various (10(-9) - 10(-6) M) concentrations has been studied on human erythroid leukaemia cell line (K562). A 14 kDa protein, which did not display any cytotoxic activity to K562 cells, but stimulate PBMC proliferation has been isolated from the supernatant of plasma free platelets stimulated by 10(-7) M Ca-ionophore. The protein displayed a very slight mitogenic effect on PBMCs, but it enhanced proliferative response of PBMC stimulated by concanavalin A at suboptimal concentrations. The p14 N-terminal sequence ((1)VLERTXA(7)-) is identical to the region 99-103 residues of the human MHC class II antigen DQ-beta chain. Also, we identified this 14 kDa protein in the supernatant of PBMCs stimulated by 10(-7) M Ca-ionophore. Its N-terminal sequence (VLERTXA-) is identical to the one of p14 from the stimulated plasma-free platelet supernatant.