Study of natural lipocortin I. A potent mediator for macrophage-mediated immunosuppression in tumor-bearing mice.

We previously reported a possible role of lipocortin I secreted from Mac-1+, -2+ macrophages in immunosuppression in tumor-bearing mice. In this study, we purified natural lipocortin I from the spleens of tumor-bearing mice and compared its immunosuppressive activity with recombinant lipocortin I produced by Escherichia coli. The culture supernatants of splenic macrophages from tumor-bearing mice suppressed the mitogenic responses in splenic lymphocytes. The culture supernatants contained higher levels of lipocortin I, but not PGE2, which was considered as a major immunosuppressive factor. Anti-lipocortin I antiserum neutralized these inhibitory activities. Natural lipocortin I purified from the spleens of tumor-bearing mice showed a potent suppressive activity, which was > or = 200 times higher than that of recombinant mouse lipocortin I at protein level. Alkaline phosphatase treatment of natural lipocortin I decreased immunosuppressive activity to the level of recombinant lipocortin I. N-glycosidase F treatment failed to decrease the immunosuppressive activities. These results suggest that the protein modification of lipocortin I, possibly phosphorylation but not glycosylation, is critical for the potent immunosuppressive activity in tumor-bearing mice.