Activation of autoreactive T-lymphocytes by cultured syngeneic glomerular mesangial cells. The capacity of intrinsic, glomerular mesangial cells (MC) to cause an autoreactive response of syngeneic lymphocytes in vitro are presented. Initial experiments demonstrated the MHC class II dependent capacity of MC to present exogenous antigen to sensitized lymph node lymphocytes (LN) and to activate naive, allogeneic LN in the absence of a nominal antigen. However, the most striking finding of the present investigation was that mouse MC (C57BL/6 or DBA/2) augmented a significant activation of naive, syngeneic lymphocytes. The extent of the proliferative lymphocyte response was comparable to that observed after stimulation with allogeneic MC. Moreover, during syngeneic coculture substantial amounts of interferon bioactivity were generated. Equipotent concentrations of rm IFN-γ were sufficient to induce class II MHC expression of mouse MC. In control experiments the macrophage cell line, IC-21 (C57BL/6), or freshly prepared DBA/2 mouse peritoneal macrophages did not elicit a syngeneic LN response. Using MC, which had not been pretreated, the MC-specific LN stimulation occurred after prolonged periods of coculture. The stimulation index (S.I.) was 9.77 after 144 hours compared with LN controls (S.I. = 1). However, a 48 hour pretreatment of MC with either rm IFN-γ alone or in combination with rh TNF-α and/or the continuous presence of rm IL-1α during coculture periods from 72 to 144 hours substantially enhanced the proliferative LN response. Analysis of non-adherent LN by flow cytometry (FACS) after 96 or 120 hours coculture with MC revealed an increased ratio of Thy1.2⁺ to B220⁺ cells with a predominant rise of L3T4⁺ T-helper cells compared to Lyt2⁺ cytotoxic T-cells. Furthermore, immune fluorescence microscopy showed that a fraction of Thy 1.2⁺ lymphoblasts adhered to MC. FACS analysis of these adherent LN after detachment demonstrated that in comparison to cocultures with untreated MC, cocultures of LN with IFN-γ/TNF-α pre-treated MC resulted in a 24.4% increase of Thy1.2⁺ cells, with 89% of these being L3T4⁺ T-helper lymphocytes. In conclusion, autoreactivity of preferentially T-helper cells to cocultured glomerular MC was shown, which may represent a useful model of T-lymphocyte dependent glomerulonephritis.