Tumor necrosis factor-α induces coordinated changes in major histocompatibility class I presentation pathway, resulting in increased stability of class I complexes at the cell surface

It is demonstrated that similar to interferon gamma (IFR-gamma), tumor necrosis factor-alpha (TNF-alpha) induces coordinated changes at different steps of the major histocompatibility complex (MHC) class I processing and presentation pathway in nonprofessional antigen-presenting cells (APCs). TNF-alpha up-regulates the expression of 3 catalytic immunoproteasome subunits-LMP2, LMP7, and MECL-1-the immunomodulatory proteasome activator PA28 alpha, the TAP1/TAP2 heterodimer, and the total pool of MHC class I heavy chain. It was also found that in TNF-alpha -treated cells, MHC class I molecules reconstitute more rapidly and have an increased average half-life at the cell surface. Biochemical changes induced by TNF-alpha in the MHO class I pathway were translated into increased sensitivity of TNF-alpha -treated targets to lysis by CD8(+) cytotoxic T cells, demonstrating improved presentation of at least certain endogenously processed MHC class I-restricted peptide epitopes. Significantly, it was demonstrated that the effects of TNF-alpha observed in this experimental system were not mediated through the induction of IFN-gamma. It appears to be likely that TNF-alpha -mediated effects on MHC class I processing and presentation do not involve any intermediate messengers. Collectively, these data demonstrate the existence of yet another biologic activity exerted by TNF-alpha, namely its capacity to act as a coordinated multi-step modulator of the MHC class I pathway of antigen processing and presentation. These results suggest that TNF-alpha may be useful when a concerted up-regulation of the MHC Glass I presentation machinery is required but cannot be achieved by IFN-gamma.