Whether the common human cancers possess tumour-specific antigens which produce an immune response in the host still remains the crucial question in tumour immunology. Recent reports and discussions at the Third International Congress of Immunology suggested that much of the previously published work was not valid. The reasons for this have been analysed by comparing the well-established experimental data with human cancer. It is suggested that much of the current confusion in tumour immunology stems from the poor models used to study human cancer and the inadequate assessment of in vitro assays which have measured the host response. The present methods require refinement, or new techniques need to be developed, before this question can be answered. The field has been well researched, and there do not appear to be any recent major advances in tumour immunology which can be applied in clinical practice.
Tumour resistance was investigated in a spontaneously metastasising murine melanoma. Specific tumour resistance was found in mice with intact small primary tumours and no metastases. If primary tumours were excised, resistance was demonstrated in mice which had metastases and in mice which did not have metastases but were given crude melanoma homogenate. No resistance could be detected in mice which had large primary tumours left intact or in mice which had small primary tumours excised four days prior to tumour challenge. These in vivo findings are compared with previously documented in vitro results.
The early onset of serum blocking of host cell‐mediated immunity was investigated in a transplantable murine melanoma model employing two in vitro assays. One of these was based on the Takasugi and Klein cytotoxicity test; the other was a new test based on the inhibition of adherence of leukocytes to a plastic surface in the presence of tumor antigen. Both tests yielded similar results, and the adherence inhibition test was more sensitive. Blocking could be detected as early as 1 h after the inoculation of 1×10 7 melanoma cells. Early onset of serum blocking was also demonstrable if homogenized or tissue‐cultured tumor cells were inoculated. Serum blocking under these circumstances was initially transient, declining at 24 to 48 h, but reappearing by the 4th day after inoculation. Furthermore, a functional immune system was not required for blocking factor development in this system, as evidenced by the onset of a similar pattern of blocking activity in heavily irradiated animals.
Peripheral blood lymphocytes from patients with a variety of tumors are capable of inhibiting growth of the appropriate target tumor cells in tissue culture (8). The specificity of reaction has been shown to be restricted to the tissue of origin of the tumor. Furthermore, serum from these patients is capable of blocking this growthinhibitory reaction and the specificity is similar to the cell-mediated immunity, (CMI) (11). Initial observations suggested that the serum-blocking factor (SBF) was antibody in nature (9) but more recent evidence demonstrated that antigen was an essential component in the syngeneic or autochthonous tumor systems studied. (4, 16). It has therefore been suggested (3, 12) that measurement of serum-blocking factor may be of value in the detection of residual tumor following primary surgical excision. Although there is evidence to suggest that loss of serum-blocking factor occurs in patients with an expected good prognosis (12), it will take some years before the value of serum-blocking factor can be adequately assessed as an index of metastatic spread following primary excision in man. The present study was designed to assess the value of serological follow-up in the postoperative period following primary tumor excision utilizing two in vitro assays of serum-blocking factor, A transplantable melanoma was chosen because it grows locally before metastasizing to the regional lymph nodes and lung. It was therefore possible to compare the serum of mice with local growth to mice with disseminated disease when the primary tumor was excised. Two methods for the measurement of serum blocking were employed: The cytotoxicity assay of Takasugi and Klein (18) and a leukocyte adherence assay. The leucocyte adherence assay is a simpler and more rapid test than the cytotoxicity assay, and preliminary work has shown that it is able to measure serum blocking with greater sensitivity (LO). It was therefore of interest to employ both tests to assess the usefulness of this simpler assay for the detection of serum-blocking factor following tumor excision and to attempt to relate the results to prognosis.
Individual mice were monitored for serum blocking factor (SBF) activity prior to and after excision of their primary spontaneously metastasing melanomas. SBF activity was measured by the leukocyte adherence inhibition (LAI) microtest. Mice which maintained high levels of SBF activity on the third day after excision subsequently died with metastases in the lung. In contrast, mice which exhibited markedly decreased SBF activity on the third day post-excision did not develop metastases. The data are discussed in terms of the efficacy of SBF monitoring as a prognostic index in cancer.
Changes in cell mediated and humoral immunity following the excision of a transplantable melanoma growing in the footpad of its syngeneic host, as measured by an in vitro cytotoxicity test, were assessed. Spleen cell cytotoxicity did not change significantly. Cells from the regional lymph nodes stimulated tumour growth before tumour excision. Three days following tumour excision this stimulatory effect was undetectable. Loss of serum factors capable of blocking the cytotoxicity of spleen cells occurred 24 h after tumour excision. Serum cytotoxicity increased after tumour excision to a maximum of the third day. Following tumour excision the rise in serum cytotoxicity and loss of regional lymph node tumour stimulation were concomitant with the loss of blocking activity.
The effect on tumour growth of adding heavily irradiated cells to a standard number of viable mouse melanoma tumour cells before its inoculation into the syngeneic host was investigated. It was found that admixed irradiated syngeneic tumour cells stimulated tumour growth. Irradiated syngeneic fibroblasts had no effect. Irradiated tumour cells with a strong histocompatability difference inhibited tumour growth. It is possible that this inhibitory effect was due to a stimulation of the host immune system.
The effect on tumour growth of adding heavily irradiated cells to a standard number of viable mouse melanoma tumour cells before its inoculation into the syngeneic host was investigated. It was found that admixed irradiated syngeneic tumour cells stimulated tumour growth. Irradiated syngeneic fibroblasts had no effect. Irradiated tumour cells with a strong histocompatability difference inhibited tumour growth. It is possible that this inhibitory effect was due to a stimulation of the host immune system.
Humoral immune response to the B16 melanoma in its syngeneic host, the C57BL/6J mouse, in volved production of complement-fixing, cytotoxic, and reaginic antibodies to B16 antigens. Lymphocytes that were stimulated to DNA synthesis when cultured with B16 antigens were present in the spleen and regional lymph nodes. Lymphocytes capable of inhibiting B16 growth in vitro were in the spleen but not in the regional lymph nodes during tumor growth; in contrast, lymphocytes of the regional nodes stimulated B16 growth in vitro. Serum collected during tumor growth converted from inhibitory to stimulatory the action of immune spleen lymphocytes on 816 growth. Macrophages in the tumor increased in number during tumor growth; histopathologic examination of the growing tumor revealed an early, transient, lymphocyte infiltrate and later macrophage involvement. The blood, spleen, and regional-node lymphocyte response to phytohemagglutinin decreased as the tumor grew. We concluded that absence of cytotoxic lymphocytes in the regional nodes, serum-mediated conversion of the effect of immune lymphocytes from inhibitory to stimulatory action on B16 growth, and anergy ofthymus-derived lymphocytes allowed the B16 tumor success over its host.
