Effects of CKS-17, a synthetic retroviral envelope peptide, on cell-mediated immunity in vivo: immunosuppression, immunogenicity, and relation to immunosuppressive tumor products

CKS-17 is a heptadecapeptide corresponding to a region highly conserved in retroviral transmembrane proteins such as p15E. Because a relationship had previously been determined between p15E and immunosuppressive tumor cell products, we examined the effect of CKS-17, control peptides and conjugates thereof on the expression of cell-mediated immunity (delayed-type hypersensitivity, DTH) in mice. Conjugates of CKS-17 inhibited DTH reactions to sheep erythrocytes in the feet of mice. The degree of inhibition was dose-dependent. Unconjugated CKS-17 had almost no effect, and control peptide conjugates had no inhibitory effect. Immunization of mice with CKS-17 conjugates, but not with control conjugates, rendered them resistant to the depression of DTH reactions, not only by CKS-17 conjugates, but also by products of cultured tumor cells. CKS-17 conjugates, but not control conjugates, also depressed the cellular inflammatory reactions induced in mouse footpads by concanavalin A (ConA) and immunized mice against the depression of ConA reactions by products of cultured tumor cells. Injections of globulin from sera of mice immunized with CKS-17 conjugates conferred upon normal recipients resistance to the depression of footpad reactions to ConA by products of cultured tumor cells. Globulin from sera of normal mice or control immunized mice did not confer such resistance. Thus conjugates of a synthetic peptide not only mimic the immunosuppressive effects of tumor products in vivo, but can also immunize mice against those effects.

Specific tumoricidal activity of cytotoxic macrophages and cytotoxic lymphocytes

The antitumor potency and specificity of syngeneic immune peritoneal exudate cells were tested. Groups of DBA/2 mice were immunized against syngeneic SL2 tumor cells. Then 6 days after the last immunization the antitumor potency, and the specificity of the immunization reaction was tested by injecting groups of the immunized mice with 10(3) to 5 X 10(7) DBA/2 derived L1210, L5178Y, P815 or SL2 tumor cells, and injecting immune peritoneal exudate cells into DBA/2 mice which had been injected 2 h earlier i.p. with 2 X 10(4) or 2 X 10(5) L1210, L5178Y, P815, or SL2 cells. Furthermore the tumor specific cytotoxicity in vitro of isolated immune (vs SL2) peritoneal macrophages was tested against L1210, L5178Y, P815, and SL2 cells. The "reciprocal" experiments (previous immunization against L1210, L5178Y, or P815 cells and 'challenge' with SL2) were also done. Finally, we tested the tumor-specific cytotoxicity of isolated immune peritoneal T-lymphocytes. It was shown that the rejection of tumor cells in previously immunized mice, the antitumor efficacy of the transferred immune peritoneal exudate cells and the in vitro cytotoxicity of purified immune peritoneal macrophages and lymphocytes, were tumor-specific reactions. That is only between the SL2 and L5178Y tumors were cross-reactions observed. However, this cross-reaction was not found at the level of cytotoxic T-cells. This suggests that cytotoxic T-cells and cytotoxic macrophages probably have different mechanisms of recognition of the specific tumor target cells. Treatment of macrophage monolayers, prepared from macrophages of immunized mice, with monoclonal anti-Thy-1 antibodies plus complement caused no decrease in cytotoxicity. This shows that macrophages can really express specific cytotoxicity. Tumoricidal macrophages probably obtain their tumor specificity through the activities of tumor-specific factors produced by sensitized T-cells.

Eradication of tumor cells after injection into immunized hosts compared with the eradication of tumor cells after transfer of immune peritoneal exudates into tumor-bearing recipients

DBA/2 mice were immunized against the syngeneic SL2 lymphoma by two or five injections with irradiated lymphoma cells given IP or SC, respectively. The antitumor efficacy induced in immunized mice was tested by (a) IP injection of the immunized mice with nonirradiated tumor cells and (b) transfer of the total 'immune' peritoneal exudate, the cellular fraction only, or the cell-free fraction only, IP into tumor-bearing recipients, or (c) tumor neutralization tests (Winn assay). It was shown that immunized mice were able to reject 5 X 10(7) SL2 tumor cells (8 of 14 mice survived greater than 100 days), while in most transfer experiments 2 X 10(5) SL2 cells could be eradicated. In the tumor neutralization experiments a number of 10(6) SL2 cells were eradicated. When the immune exudates were given before the inoculations of SL2 tumor cells the number of survivors increased significantly. Further, it was shown that the cellular fraction is the major contributor to the antitumor effect in the transfer experiments, since there was no significant difference in tumor eradication after injection of a complete immune exudate and after injection of the isolated cellular fraction. Injection of the noncellular fraction had no measurable antitumor effect. An increase in the number of injections with total peritoneal exudates from immunized mice did not result in an increase in tumor eradication in the tumor-bearing recipients. Extra stimulation (IP) of immunized mice with 10(4) nonirradiated cells 6 days after the last immunization resulted in an increase of the antitumor efficacy of these peritoneal exudates of these mice when collected 4-24 h after this stimulation. Extra stimulation with 10(6) irradiated cells had no measurable effect.