Depletion of macrophages expressing I-J antigen results in efficient generation of alloreactive cytotoxic T lymphocytes

The role of suppressor macrophages (S-Mφ) produced during generation of cytotoxic T lymphocytes (CTL) stimulated with allogeneic lymphocytes was investigated. Splenic CTL from C3H/He mice (H-2^k) were generated by in vivo immunization and subsequent in vitro stimulation by splenic lymphocytes from C57B1/6 mice (H-2^b) in mixed lymphocyte reaction (MLR). In addition to in vitro standard ⁵¹Cr release assay, the CTL activity was mainly measured in vivo using the Winn assay against EL-4 thymoma cells in B6C3F₁ mice (H-2 ^{b k}). In mice injected with CTL plus EL-4 cells survival rate was 20% compared with no survival of mice treated with normal spleen cells plus EL-4 cells. The antitumor activity of the CTL was significantly increased when immunized mice were treated with a 5 mg/kg ip dose of indomethacin at the time of immunization (80% survival). Macrophages were depleted from spleen cells of immunized mice by plastic adherence or carbonyl-iron treatment, replaced with an equivalent number of Mφ from normal mice, and then introduced into a 5-day MLR. When the antitumor activity of the cells isolated from this MLR was measured in the Winn assay, 90-100% survival in EL-4-bearing mice was observed. In contrast, none of the mice inoculated with EL-4 alone and 20% of the mice that received CTL obtained after alloimmunization followed by MLR in addition to EL-4 survived. These results of CTL activity were confirmed by in vitro cytotoxicity tests. When the Mφ isolated from spleens of immunized mice were analyzed for I-J^k antigen expression, a 2.5-fold increase was detected, compared with splenic Mφ obtained from normal C3H/He mice. In contrast, Ia and I-A^k antigen expression was equivalent in Mφ isolated from normal or immunized C3H/He mice. When immune spleen cells were treated with anti-I-J^k antiserum followed by complement and then, subjected to the MLR, the antitumor activity of CTL was significantly enhanced (80% survival). However, treatment of these cells with anti-I-A^k antiserum and complement did not alter CTL activity. These data suggest that the increase of S-Mφ expressing I-J^k+ antigen to be induced during alloimmunization results in suppression of allospecific CTL-generation in MLR.