T cells which do not express membrane tumor necrosis factor-alpha activate macrophage effector function by cell contact-dependent signaling of macrophage tumor necrosis factor-alpha production.

Previous studies have suggested that T cell contact-dependent signaling of macrophages (Mphi) is mediated by membrane tumor necrosis factor-alpha (memTNF-alpha), based on the observation that anti-TNF-alpha could inhibit T cell-mediated Mphi activation. The current report confirms that anti-TNF-alpha does inhibit activation of interferon-gamma (IFN-gamma)-primed Mphi by paraformaldehyde-fixed activated T cells. However, the involvement of membrane molecules other than memTNF-alpha in the contact-dependent signaling is suggested by two lines of evidence. First, the TH2 clone, AK8, displayed neither secreted TNF-alpha/beta nor memTNF-alpha/beta detectable by bioassay or immunofluorescence. Nonetheless, AK8 cells were equally effective, on a per cell basis, in contact-dependent signaling of M phi activation as TH2 and TH1 cells which do express memTNF-alpha. Second, the expression of memTNF-alpha by the TH2 clone, D10.G4, is maximal 24 h after activation, whereas the ability of this clone to activate Mphi is maximal at 6-8 of activation and declines thereafter. Since TNF-alpha is known to play a critical role in activation of Mphi effector function, it was hypothesized that T cell membrane components other than memTNF-alpha might signal Mphi production of TNF-alpha, thus allowing autocrine TNF-alpha stimulation of Mphi effector function. In support of this, it is demonstrated that paraformaldehyde-fixed activated TH2 cells can induce de novo production and release of TNF-alpha by Mphi. This effect was not an artifactual result of paraformaldehyde fixation since paraformaldehyde-fixed resting T cells did not induce TNF-alpha gene expression. Previous studies have demonstrated a role for autocrine TNF-alpha stimulation in LPS induction of effector function in recombinant IFN-gamma-primed Mphi. The current study confirms that TNF-alpha plays a critical role in T cell contact-dependent signaling of Mphi but indicates that memTNF on the T cells may not be a sine qua non factor for contact-dependent signaling. The data suggest that other T cell membrane molecules contribute to activation of Mphi effector function by stimulation of M phi TNF-alpha production.