Thymic epithelial cell lines (TECs) were established from newborn C57BL/6 mice. They were classified into two types (medullary and cortical TECs) by using the monoclonal antibody (Th-3) that recognizes the meshwork structure of thymic cortical epithelial cells. Antigen-presenting activity of each TEC was determined by using ovalbumin-specific, I-Ab-restricted helper T cell lines. It was demonstrated that the medullary but not the cortical TECs functioned as antigen-presenting cells. This is the first evidence for the functional difference between the cortical and the medullary TEC.

A mouse monoclonal antibody (IgM) was obtained by cell hybridization between X63-Ag8.653 myeloma cells and spleen cells from a BALB mouse that was immunized with GRSL leukemia cells of the GR strain. This antibody identified a unique fetal antigen, which is expressed exclusively on embryonic thymocytes of all strains tested. Therefore, the antigen defined was named fetal thymus antigen-1, FT-1. The proportion of FT-1+ fetal thymocytes detected by immunofluorescence assay sharply decreases as gestation time increases, and finally they disappear from the thymus. On the other hand, Thy-1+ cells increase in inverse proportion. The immunofluorescence studies and absorption tests showed that FT-1 antigen is not detectable on brain, liver, kidney, or lymphoid tissue cells of adult mice. However, it is expressed on some leukemia cells of various mouse strains, which demonstrated that this is the first example of an oncofetal antigen of a mouse leukemia. The molecular weight of FT-1 antigen on leukemia cells was estimated to be 130,000 by means of biosynthetic labeling with [3H]galactose and [35S]methionine. The two-dimensional gel electrophoresis pattern of FT-1 antigen shows a family of glycoproteins with extensive charge heterogeneity. It was also shown that the FT-1 antigen molecule carries the receptor for DBA lectin.

Relatively large numbers of nonimmune spleen cells do not protect against the local growth of two lymphomas. However, this heterogeneous population of splenic lymphocytes contains a subset of cells that efficiently protects against in vivo tumor growth. This cell population (cell-surface phenotype Thyl.2(-)Ig(-)Ly5.1(+)) represents less than 5 percent of the spleen cell population and is responsible for in vitro NK-mediated lysis. Although these studies clearly and directly demonstrate that Ly5(+) NK cells selected from a heterogeneous lymphoid population from nonimmune mice can protect syngeneic mice against local in vivo growth of two different types of tumor cells (in contrast to other lymphocyte sets within the spleen), they do not directly bear upon the role of NK cells in immunosurveillance. They do indicate that highly enriched Ig(-)Thyl(-)Ly5(+) cells, which account for virtually all in vitro NK activity, can retard tumor growth in vivo. It is difficult to ascribe all anti-tumor surveillance activity to NK cells, because they probably do not recirculate freely throughout the various organ systems of the body. Perhaps NK ceils may play a role in prevention of neoplastic growth within discrete anatomic compartments where there is rapid differentiation of stem cells to mature progeny (e.g., bone marrow, spleen, and portions of the gastrointestinal tract)and may normally act to regulate the growth and differentiation of non-neoplastic stem cells. Long-term observation of chimeric mice repopulated with bone marrow from congenic or mutant donors expressing very low or very high NK activity may help to answer these questions.