Soluble, but Not Transmembrane, TNF-α Is Required during Influenza Infection To Limit the Magnitude of Immune Responses and the Extent of Immunopathology

TNF-alpha is a pleotropic cytokine that has both proinflammatory and anti-inflammatory functions during influenza infection. TNF-alpha is first expressed as a transmembrane protein that is proteolytically processed to release a soluble form. Transmembrane TNF-alpha (memTNF-alpha) and soluble TNF-alpha (solTNF-alpha) have been shown to exert distinct tissue-protective or tissue-pathologic effects in several disease models. However, the relative contributions of memTNF-alpha or solTNF-alpha in regulating pulmonary immunopathology following influenza infection are unclear. Therefore, we performed intranasal influenza infection in mice exclusively expressing noncleavable memTNF-alpha or lacking TNF-alpha entirely and examined the outcomes. We found that solTNF-alpha, but not memTNF alpha, was required to limit the size of the immune response and the extent of injury. In the absence of solTNF-alpha, there was a significant increase in the CD8(+) T cell response, including virus-specific CD8 + T cells, which was due in part to an increased resistance to activation-induced cell death. We found that solTNF-alpha mediates these immunoregulatory effects primarily through TNFR1, because mice deficient in TNFR1, but not TNFR2, exhibited dysregulated immune responses and exacerbated injury similar to that observed in mice lacking solTNF-alpha. We also found that solTNF-alpha expression was required early during infection to regulate the magnitude of the CD8(+) T cell response, indicating that early inflammatory events are critical for the regulation of the effector phase. Taken together, these findings suggest that processing of memTNF-alpha to release solTNF-alpha is a critical event regulating the immune response during influenza infection.