We studied the plasmin system with specific antisera to plasminogen, its 2 activators (urokinase-type and tissue-type) and the 2 plasmin inhibitors, alpha 2 anti-plasmin and alpha 2 macroglobulin on sections of 34 human colonic tumors by immunofluorescence. Anti-plasminogen serum showed a clear-cut reactivity at the surface of tumor cells, as it stained the contour of tumor glandular structures, foci, and isolated tumor cells. Intratumoral deposits and necrotic areas were stained as well, often strongly. Localization of plasminogen was quite different from that of fibrinogen, which was found only in peritumoral stroma, and never on tumor cells. Traces of both types of plasminogen activator were found, mainly on invasive tumor cells for urokinase type and on large tumor foci for tissue type. Images were weak and inconstant. Large amounts of both plasmin inhibitors were characterized in tumor stroma. Alpha-2 anti-plasmin was also found in intratumoral deposits and necrotic areas. It seems likely that plasminogen exudes from blood capillaries (since anti-plasminogen serum often stained the whole capillary wall), diffuses in the stroma and binds to tumor cells. Once formed, plasmin is likely to play a role in the invasion of surrounding tissues by tumor cells, in the dissociation of tumor cells from tumor glands and in the production of necrosis inside tumor areas.

Antigens of the basement membrane (type-IV collagen and laminin) and the connective tissue (type-III collagen and fibronectin) were studied by immunofluorescence in 16 lymph nodes draining colorectal carcinomas and 6 lymph nodes draining breast carcinomas. A comparison was also made between 7 primary colorectal carcinomas and 9 lymph nodes draining these tumors. Anti-type-IV collagen and anti-laminin rarely stained the basement membrane of metastatic tumors. In contrast, we detected type-IV collagen in the peritumoral stroma, although similar images were rarely seen in primary tumors. When tumoral cells were in the vicinity of lymphoid cells, they were occasionally separated by a barrier stained by the four antisera, or only by antifibronectin and anti-type-III collagen. In other cases no barrier was observed between both types of cells which were in close contact. On the whole the above alterations were more marked in the lymph nodes draining breast carcinomas, in comparison to those draining colorectal carcinomas. Tumor cells were never stained by anti-type-IV collagen or antilaminin serum. Some cells found either in the lymphoid or in the tumor area of metastatic lymph nodes were stained not only by these antisera, but also by a monoclonal antibody against Willebrand Factor, which is a marker of endothelial cells. Thus the labelled cells were characterized as being derived from the capillary wall.

The morphologic responses of the lymphoid system of the rabbit were studied. The results demonstrated: (a) the proliferation of a characteristic cell, the "hemocytoblast", within lymphoid centers; (b) architectural changes in lymph nodes and spleen, at first characterized by enlargement of follicles and germinal centers, later by effacement of normal landmarks and finally by reconstitution of lymphatic structures; (c) a barely perceptible plasmocytic response, especially in the first set reaction. These changes, although most pronounced in the lymph node proximal to the homograft, were also found later in distant lymph nodes and in the spleen. They arose more rapidly, were of greater intensity and were more persistent in the second set reaction than in the first set reaction. These findings indicated that transplantation immunity eventually invokes generalized morphologic changes in the lymphoid system. The events which followed a second set graft suggested that the second set phenomenon had the features of an anamnestic response.

The effects of 6-mercaptopurine and thioguanine on the morphologic responses of lymph nodes and spleen were studied in the rabbit. The main effect of these drugs was to inhibit the proliferation of hemocytoblasts, cells considered to be immunologically competent. Homograft rejection did not occur as long as these cells were suppressed. Rabbits which rejected their grafts in spite of continued 6-mercaptopurine treatment developed large numbers of hemocytoblasts in their lymphoid apparatus. These cells were thought to be drug resistant and analogous to resistant leukemic cells.