Facilitated tumor growth was observed in syngeneic animals after the subcutaneous inoculation of these animals with normal syngeneic spleen cells a n d tumor cells previously mixed in vitro. T u m o r incidence, latency, a n d growth rate appear quantitatively directly related to the ratio of spleen cells to tumor cells in the challenge inocular. This phenomenon was consistently denionstrated i n two syngeneic murine tumor-host systems a n d o n e sygeneic rat system. Facilitated tumor growth is not a specific property of the spleen cells i n these mixtures. Mixtures of liver cells and tumor cells ~~r o d u c e identical results. Reproducible facilitated growth of tumor isografts by mixture with normal spleen cells, however, must he considered in the design a n d interpretation of all in vivo neutralization experiments.
SOGRAFTS OF HYDROCARRON-INDUCED TIJMORS I <
?row progressively in syngeneic hosts, and, in the absence of specific treatment, death of the host inevitably results.10 These tumols possess tumor-specific transplantation anti-Vens4. h 6 . 12-15 capable of initiating a specific immunologic reaction of the host directed against its own primary tumo'r.5-6 , 9, However, this immune response within the autochthonous host will not normally prevent eventual growth of the tumor isopraft.20 Despite these considerations, syngeneic hosts can be effectively immunized against subsequent tumor isoprafts by the excisioii of growing tiimors.13, 14 This immunity is mediated by lymphoid cells.13 I t can be adoptively transferred by these cells,2. 3. 13 but not by isologous immune sera. Such sera, however, when mixed with syngeneic tumor cells, have often caused enhancement of tumor growth.11 Generally, therefore, tumor cell-lymphoid cell interactions are responsible for inhibition of tumor growth, while immunologic enhancement may be a consequence of the contact of tumor cells with specific isoantibodies.7
Numerous reports have shown that when immune lymphoid cells are mixed, in vitro, ____