VCAM-1 influences Lymphocyte Proliferation and Cytokine Production during Mixed Lymphocyte Responses

The allogeneic mixed lymphocyte reaction (MLR) is regarded as an effective model for examining the events which occur during an allospecific immune response. Numerous studies have delineated the role of adherence molecule interactions during the MLR response. In the present study we have identified VCAM-1 as having a contribution to the generation of an allogeneic MLR response. These findings may have broad implications in vivo during antigen-specific and allograft rejection events. RT-PCR analysis was initially used to examine whether VCAM-1 mRNA expression was observed during MLR responses and demonstrated peak expression between 12 and 48 hr of culture. Immunolocalization of VCAM-1 on adherent mononuclear phagocytes, but not non-adherent lymphocytes, from MLR cultures verified its expression during this response. Addition of anti-VCAM-1 mAbs to MLR assays inhibited the proliferative response by over 70%, while addition of anti-VCAM-1 as late as Day 2 of the assay allowed significant inhibition of the proliferative response. This correlated with peak expression of VCAM-1 mRNA observed as late as 48 hr in RT-PCR analyses. In further studies, anti-VCAM-1 significantly inhibited peak expression of IL-2 on Days 3 and 4, while TNF-α production was diminished at 30 min and 1, 96, and 120 hr of culture, compared to control cultures. The production of macrophage inflammatory protein-1α (MIP-1α), a chemotactic cytokine which has an important role in vivo for the recruitment of leukocytes to a site of inflammation, was also significantly inhibited during peak production on Days 4 and 5 of the MLR assay. This study demonstrates novel findings of VCAM-1 expression during an allogeneic MLR response. The expression of VCAM-1 may have important implications during allospecific immune responses for the activation and proliferation of T lymphocytes as well as cytokine production.