Evidence that antisera that react with products of the human HLA-DR locus may block in vitro antigen-induced proliferation by inducing suppression.

It is widely recognized that antisera that interact with determinants encoded by the Ia region of the mouse and its counterpart, the DR locus of man, are capable of interfering with the ability of monocytes and lymphocytes to respond in vitro to an antigenic stimulus. Using an in vitro assay that measures antigen-specific proliferation of human lymphocytes, we found that both a heteroantiserum raised in rabbits (anti-P29,34) and alloantisera which recognize determinants encoded for by the human DR locus dramatically block in vitro antigen-specific proliferation. These anti-DR antisera appear to act at the level of the monocyte; monocytes pulsed and washed free of excess antisera fail to promote proliferation in the presence of soluble antigen and untreated T cells whereas identically pulsed and washed T cells respond normally with untreated monocytes. Furthermore, the addition of unpulsed monocytes fails to restore in vitro antigen-specific reactivity. Our data suggest that membrane-bound anti-DR-specific antisera on monocytes is profoundly suppressive, and also suggests simple steric hindrance may not account for all of the observed effects such antisera have on monocyte-T cell interactions.