ROLE OF TUMOR-NECROSIS-FACTOR-ALPHA IN DENDRITIC CELL-MEDIATED PRIMARY MIXED LEUKOCYTE REACTIONS

Tumour necrosis factor (TNF alpha) is a major inflammatory cytokine wi th potentiating effects on specific immune responses, including graft- versus-host disease. This study examined the contribution of TNF alpha to dendritic cell (DC)-mediated primary allogeneic T lymphocyte respo nses, Purified blood DC were shown to produce minimal amounts of TNF a lpha mRNA but no significant TNF biological activity or secreted TNF a lpha as measured by ELISA. Amplification of DC mRNA by PCR using oligo nucleotide primers to CD120a (TNFRI, p55) and CD120b (TNFRII, p75) and probing with specific internal oligonucleotides, suggested that DC ex press the CD120b but little if any CD120a. These results were confirme d using monoclonal antibodies to the TNF receptors. Polyclonal antiser um specific for TNF alpha blocked the blood DC-stimulated allogeneic m ixed leucocyte reaction (MLR), The addition of TNF alpha to suboptimal MLRs (limited DC stimulators), increased the proliferation of respond ing T lymphocytes. Having confirmed that T lymphocytes produce TNF alp ha and express CD120b after stimulation, we sought to clarify whether the contributing effect of TNF alpha to the allogeneic MLR resulted fr om a TNF alpha-mediated signal stimulating DC activity, or as a result of autocrine stimulation of T lymphocytes. Pre-incubation of DC with TNF alpha did not increase DC stimulatory capacity; and late addition of anti-TNF serum (up to 72h) still had a significant inhibitory effec t on the MLR, We conclude that TNF alpha is probably not involved in t he initial DC-T lymphocyte interaction, but acts as an autocrine growt h factor for DC induced T lymphocyte proliferation.