CHARACTERIZATION OF TNF RECEPTORS ON HUMAN THYMOCYTES

Although tumor necrosis factor-alpha (TNF) is constitutively expressed in human and mouse thymus, the effects of TNF on thymocyte proliferat ion, differentiation and survival suggest that its influence in the th ymus is complex. To determine if this complexity results from changes in the expression of the two TNF receptors during thymocyte differenti ation, we examined the expression of the 55 kDa TNF receptor (TNF-R1) and the 75 kDa TNF receptor (TNF-R2) on postnatal human thymocytes. Bo th TNF-R1 and TNF-R2 mRNA were found in resting human thymocytes by re verse transcriptase-polymerase chain reaction (RT-PCR). Using mAb whic h specifically react with the respective TNF receptors and a highly se nsitive, three-step method of immunofluorescence, cell surface TNF-R1 was detected on the vast majority of thymocytes. In contrast, detectab le cell surface TNF-R2 was present on a mean of only 12.9% of thymocyt es. TNF conjugated to phycoerythrin (TNF-PE) also reacted with a small population of thymocytes and was found to specifically block binding of the TNF-R2 mAb and not the TNF-R1 mAb, implicating preferential bin ding of TNF-PE to TNF-R2. Using dual-color immunofluorescence with TNF -PE we found that the population of cells which express TNF-R2 also ex press high levels of the TCR alpha,beta-CD3 complex, CD4 or CD8, and I L-2 receptor alpha chain. Thus, immature (TCR(neg/low)) thymocytes exp ress TNF-R1 while mature (TCR(high)) thymocytes can also express TNF-R 2. This differential expression of TNF receptors provides a mechanism for distinct effects of TNF on immature vs. mature thymocytes.