We have previously demonstrated that depletion of CD8+ T-cells by the use of a monoclonal anti-Lyt-2.2 antibody abolishes the curative effectiveness of low-dose
melphalan (
L-phenylalanine mustard;
L-PAM)
therapy for BALB/c mice bearing a large (greater than or equal to 20 mm) s.c. MOPC-315
tumor and extensive
metastases (Mokyr et al.,
Cancer Res., 49: 4597-4606, 1989). Here we show that as a consequence of low-dose
L-PAM therapy, CD8+ T-cells accumulate in the s.c.
tumor nodules of MOPC-315
tumor bearers. Specifically, an 80-fold increase in the number of CD8+ T-cells was seen within 5 days after the
chemotherapy. Treatment of MOPC-315
tumor bearers with low-dose
L-PAM in conjunction with monoclonal anti-Thy-1.2 or anti-Lyt-2.2 antibody, in contrast to treatment with monoclonal anti-L3T4 antibody, prevented the appearance of the massive CD8+ T-cell infiltrate in the s.c.
tumor nodules. Fresh CD8+ T-cells derived from s.c. MOPC-315
tumor nodules that were regressing as a consequence of low-dose
L-PAM therapy exhibited a potent direct lytic activity against the MOPC-315
plasmacytoma in a short-term in vitro assay. The specificity of the lytic activity exhibited by the CD8+ T-cells was illustrated not only by the inability of the CD8+ T-cells to lyse two antigenically unrelated
thymomas (the WEHI 22.1 and the EL-4) and a natural killer-sensitive
lymphoma (the YAC-1), but also by their relatively weak lytic activity against an antigenically related
plasmacytoma (the MOPC-104E). Thus, CD8+ T-cells that infiltrate the s.c.
tumor nodules of MOPC-315
tumor bearers following low-dose
L-PAM therapy most likely exploit a CTL-type lytic mechanism to eradicate at least part of the large
tumor burden not eliminated by the direct antitumor effects of the
drug.