Suntmao'. Mifc bearinR prnjjrcssively grnwinji isoRrafts of mrtliylcholanthn-neindiict'd luiinmrs resisted tlif growth of tells of thu .saim-luinour Jiijri-tfJ into the footpad. Resistanw was manift-sted in two phast's. In ihi-first phase, rcsistante was sptfific-for the original tumour. In t\u-scttmd phasi-. ihc miL-e wenresistant lo other lines of syngeneic niethylchoIaiithrene-indiKH'd tmnours. Micr from which the finit tumour was completely exci.setl (luring the devebpiiienl of the first phiise hud lost their n'sistance when challenged 5 days later. Mice fmui which thf tmnijurs liad lieen iiicomplftely excised were as resistant as miee witli intact tumours. It is concluded that the resistanee of tuniour-hearirig mice to footpad isografts of tumours Is an example of concomitant iinmunitj.INTRODUCTION.

Summary Evidence is produced to show that two specific subclasses of IgM antibodies are formed during the primary immune response to Type III pneumococcal polysaccharide (SIII) in mice. The IgM proteins can be divided into two groups based on different reactivities with protein‐A. As with human IgM subclasses, it is proposed to call the two IgM subclasses IgM1 and IgM2, where the latter is defined by the ability to react with protein‐A of Staphylococcus aursus. Only the IgM molecule which reacted with protein‐A produced passive haemolysis in the presence of guinea‐pig complement. Results also show that the IgA anti‐SIII activity in serum of SIII‐immune mice on day 5 of the primary response is due lo hybrid IgM/A(κ) antibody which is undetected by conventional methods for enumerating antibody‐forming cells because cells producing IgM/A antibody develop direct plaques with guinea‐pig complement. A monoclonal IgM/A(κ) anti‐SIII produced from spleen cells 5 days after injection of 10 μg SIII plus pertussis vaccine exhibited similar properties to molecules with μ and α‐determinants in serum. The IgM/A hybrid antibody reacted strongly with protein‐A and produced passive haemolysis of SIII‐coated erythrocytes in the pretence of guinea‐pig, but not mouse, complement Despite its specificity for the capsular antigen of Type III pneumococci, the IgM/A hybrid antibody conferred only temporary immunity in mice challenged with viable pneumococci. The nature and properties of IgM/A antibodies as well as those of the subclasses of IgM may give important clues lo the genetic regulation and expression of antibody production. These findings may provide an explanation for some of the anomalies in various areas of immunological research.

The occurrence of DNA synthesis in mouse peritoneal macrophages was estimated autoradiographically in cells cultured on glass in the presence of tritiated thymidine. Subcutaneous injection of a soluble antigen, human serum albumin (HSA) into mice previously immunized with HSA in Freund’s complete adjuvant was followed, after a lag of at least 16 h, by the onset of DNA synthesis in peritoneal macrophages. This effect was immunologically specific and was accompanied by morphological changes indicating activation. The ability to respond in this way was passively transferable by means of spleen cells, but not serum, from immunized mice. DNA synthesis in macrophages was induced in vivo by Concanavalin A, phytohaemagglutinin-P, anti-lymphocyte globulin, bacterial endotoxin and normal mouse serum. The number of macrophages synthesizing DNA was also increased in mice with graft-versus-host reactions and in mice bearing a syngeneic methylcholanthrene-induced tumour.

Immune resistance to methylcholanthrene-induced tumors has two phases, an early specific and a late non-specific phase. Both phases were found to be T-cell-dependent in vivo. Thus, adult thymectomized, irradiated, bone-marrow-protected mice bearing H-1 tumor isografts showed impaired resistance to challenge with homologous (H-1) and heterologous (H-3) tumor cells. In each case resistance was restored by injection of thymus cells. In vitro analysis of the cellular basis of resistance revealed that different mechanisms were involved in the two phases. The cytotoxic effect of immune spleen cells taken during the early specific phase was inhibited by pretreatment with anti-O serum and complement and by removal of macrophages. Neither procedure, however, interfered with the cytotoxic potential of immune spleen cells taken during the late non-specific phage of immunity. Passage of immune spleen cells through rabbit-IgG anti-mouse immunoglobulin-coated columns (which yielded a T-cell-enriched, B-cell-depleted population) resulted in abrogration of cytotoxicity whether the cells were obtained during the early or the late phase of resistance. The inability of late-phase spleen cells to kill was explicable in terms of B-cell removal since T-cells and macrophages had been shown to be ineffective at that time. In contrast, the failure of column-treated cells from the early phase to kill was found to be due to removal of adherent cells rather than B-cells since cytotoxicity (1) was abrogated by passage through control columns coated with rabbit-IgG anti-sheep red blood cell antibody which did not retain B-cells and (2) could be restored by addition of immune macrophages (from anti-O serum-treated spleens). Taken together, these results indicate that the cellular basis of immune resistance to methylcholanthrene-induced tumors is heterogeneous. The early specific phase seems to be mediated by an interaction between T-cells and macrophages; the late none-specific phase, although T-cell dependent in its induction, depends on a different effector mechanism, possibly involving a cell or its products of the B lineage.