Identification of Tumor-Infiltrating Macrophages as the Killers of Tumor Cells After Immunization in a Rat Model System

Abstract

Immunization can prevent tumor growth, but the effector cells directly responsible for tumor cell killing in immunized hosts remain undetermined. The present study compares tumor grafts that progress in naive syngeneic rats with the same grafts that completely regress in hosts preimmunized with an immunogenic cell variant. The progressive tumors contain only a few macrophages that remain at the periphery of the tumor without direct contact with the cancer cells. These macrophages do not kill tumor cells in vitro. In contrast, tumors grafted in immunized hosts and examined at the beginning of tumor regression show a dramatic infiltration with mature macrophages, many of them in direct contact with the cancer cells. These macrophages are strongly cytotoxic for the tumor cells in vitro. In contrast to macrophages, tumor-associated lymphocytes are not directly cytotoxic to the tumor cells, even when obtained from tumor-immune rats. However, CD4+ and CD8+ T cells prepared from the regressing tumors induce tumoricidal activity in splenic macrophages from normal or tumor-bearing rats and in macrophages that infiltrate progressive tumors. These results strongly suggest that the main tumoricidal effector cells in preimmunized rats are macrophages that have been activated by adjacent tumor-immune lymphocytes.

Increased primary tumor growth in mice null for β3- or β3/β5-integrins or selectins

“…The same could be true for their infiltration of tumors, and we do detect fewer macrophages infiltrating tumors growing in integrin-or selectin-deficient mice, although the numbers of circulating monocytes͞macrophages do not differ among the strains of mice (15). Macrophages have been suggested to be tumoricidal (36)(37)(38)(39), although other reports have suggested that they also can contribute to tumor growth (40-42); they probably have both effects (42)(43)(44)(45)(46).…”

Selective depletion of inducible HSP70 enhances immunogenicity of rat colon cancer cells

“…In vivo studies have shown that activated macrophages exhibit tumor cell cytotoxicity (Hibbs et al, 1987) and that macrophage in®ltration is an important phenomenon of the host response to tumor rejection (Shrestha et al, 1999). Further, we have recently observed that T-cell activated, tumor-in®ltrating macrophages play an important role in the elimination of parental PRO tumors engrafted in animals previously injected with their immunogenic clone (Bonnotte et al, 2001). We show here that HSP70 depletion makes PRO cells susceptible to macrophage mediated cytolysis.…”

“…Immunohistochemical analyses of tumor sections obtained at day 2 after cell injection in BDIX rats showed ED2 + , OX17 + positive, activated macrophages to be prominent in the proximity of both PRO-C and PRO-70AS tumor cell nodules ( Figure 7a). Recent observations from our group suggest that tumor-in®ltrating macrophages play a central role in tumor cell killing of PRO cells, when grafted in previously immunized rats (Bonnotte et al, 2001). Therefore, we checked whether HSP70 expression could modulate tumor cell response to macrophage-mediated cytotoxicity in an ex vivo co-culture system.…”

CD4+CD25+ regulatory T cells suppress tumor immunity but are sensitive to cyclophosphamide which allows immunotherapy of established tumors to be curative

“…4a). We previously reported that T lymphocytes from tumor-immune rats had no direct cytotoxic effect on PROb cells, but strongly stimulated the tumoricidal activity of peritoneal and tumor-infiltrating M ¶ through IFN-+ secretion [16]. In contrast to unfractioned splenic T cells from PROb tumor-bearing rats, which did not activate M ¶ -mediated cytotoxic activity, CD25-depleted T cells from the same animals induced M ¶ to kill the PROb cells (Fig.…”

“…Two clones established from the same tumor cell line were used [15]. When injected into syngeneic rats, the REGb cell line yields tumors that completely regress through a T cell-dependent immune response that leads to tumor cell killing by tumor-infiltrating M ¶ [16]. The PROb cell line yields progressive and lethal tumors, while maintaining tumor rejection antigen expression, since PROb cells are rejected by syngeneic rats immunized against REGb cells [15].…”