BALB/c mice cured of a large MOPC-315 or MOPC-104E
plasmacytoma following treatment with a low dose (2.5 mg/kg) of
melphalan (
L-PAM) were resistant to challenge with the other
plasmacytoma but to a much lesser extent than to challenge with the autochthonous
plasmacytoma. The resistance of the
L-PAM-cured MOPC-315-tumor bearers to challenge with MOPC-104E
tumor cells was increased when the MOPC-104E
tumor cells were admixed with MOPC-315
tumor cells prior to their inoculation. This enhanced resistance to MOPC-104E cells was due to elimination of the MOPC-104E
tumor cells through an innocent bystander killing effect since it did not render the mice more resistant to a subsequent challenge with MOPC-104E
tumor cells alone. Administration of
carrageenan to
L-PAM-cured MOPC-315-tumor bearers 1 day after the challenge with the mixture of MOPC-104E and MOPC-315
tumor cells drastically reduced the ability of the mice to resist the
tumor challenge. All of the
tumors that developed in such mice were of MOPC-104E origin only (as judged by the binding specificity of the
myeloma proteins secreted by the
tumor cells as well as that present on their surface) even though (a) the
tumor inoculum used consisted of up to 10-fold more MOPC-315 than MOPC-104E
tumor cells and (b) the MOPC-315
tumor cells divide more rapidly. The same protocol of
carrageenan treatment did not reduce the ability of normal BALB/c mice to develop in vivo a primary cell-mediated cytotoxic response nor a primary antibody response indicating that it has no effect on the initiation of an immune response. Therefore, it is conceivable that
carrageenan treatment reduced the ability of
L-PAM-cured MOPC-315-tumor bearers to reject a challenge with MOPC-315 and MOPC-104E
tumor cells by interfering at the effector stage. The ability of the
L-PAM-cured MOPC-315-tumor bearers to reject the MOPC-315 cells present in the challenge mixture was reduced when the mice were treated with
anti-Thy 1.2 antibody but not with
carrageenan, indicating that T-cells independent from
carrageenan-sensitive effector cells are required for the rejection of the MOPC-315
tumor cells. Thus, at least two different effector mechanisms participate in the rejection of a challenge composed of MOPC-315 and MOPC-104E
tumor cells by
L-PAM-cured MOPC-315-tumor bearers.(ABSTRACT TRUNCATED AT 400 WORDS)