The intrathymic differentiation process by which precursor cells derived from the bone marrow develop into immuno-competent T lymphocytes is poorly understood. Most thymocytes express both CD4 and CD8 accessory molecules, yet little is known about either the function of these molecules or the responsiveness of the CD4+8+ double positive thymocytes that bear them. Here, we address the possibility that CD4 engagement influences T-cell receptor (TCR) expression on developing thymocytes. We engaged CD4 molecules on murine thymocytes by in vivo injection of an anti-CD4 monoclonal antibody, which reduced the surface expression of CD4 on CD4+ thymocytes. More importantly, CD4 engagement also affected TCR expression on CD4+ thymocytes, but the effect on CD4+8+ double positive and CD4+8- single positive thymocytes was very different. CD4+8+ thymocytes responded to CD4 engagement by dramatically increasing surface expression of TCR, whereas CD4+8- thymocytes decreased surface expression of TCR. These results demonstrate that the effect of CD4 engagement on TCR expression is dependent upon the developmental state of the responding thymocyte, and, most interestingly, results in increased TCR expression by double positive thymocytes.

In order to investigate the role of T cell receptor (TcR) expression in thymocyte maturation, we have analyzed thymocytes from C.B-17/SCID mice, which are unable to productively rearrange their antigen receptor genes and fail to express TcR. Despite this defect, SCID thymocytes are functional as they produce lymphokines and proliferate in response to a variety of stimuli. Phenotypic analysis revealed that thymocyte populations from young adult SCID mice resemble thymocyte populations from normal embryonic mice in that they are large, Thy-1.2+, CD4-, CD8-, TcR- and enriched in CD5lo, IL2R+ and Pgp1+ cells. However, other TcR- populations normally present in adult mice (i.e., CD4-CD8+ cells and CD4+CD8+ cells) are absent from the thymus of TcR- adult SCID mice. To understand the basis of the developmental arrest of TcR- SCID thymocytes at the CD4-CD8- stage of differentiation, we analyzed thymi from the occasional "leaky" SCID mouse which possesses small numbers of TcR+ thymocytes. We found that the presence of TcR+ cells within a SCID thymus was invariably associated with the presence of CD4+ and/or CD8+ SCID thymocytes. Interestingly, however, the CD4+/CD8+ SCID thymocytes were not themselves necessarily TcR+. That is, emergence of SCID thymocytes expressing CD4/CD8 was tightly linked to the presence of TcR+ cells within that SCID thymus, but the SCID thymocytes that expressed CD4/CD8 were not necessarily the same cells that expressed TcR. Finally, we found that the introduction into TcR- SCID mice of normal bone marrow cells that give rise to TcR+ cells within the SCID thymus promoted the differentiation of SCID thymocytes into CD4-CD8+ and CD4+CD8+ TcR- cells. These data indicate that TcR+ cells within the thymic milieu provide critical signals which promote entry of CD4-CD8-TcR- precursor T cells into the CD4/CD8 differentiation pathway. When applied to differentiation of normal thymocytes, these findings may imply a critical role for early appearing CD4-CD8- TcR (gamma/delta)+ cells in initiating normal thymic ontogeny.

Immunotherapy utilizing the adoptive transfer of lymphokine activated killer (LAK) cells in conjunction with recombinant interleukin-2 (IL-2) is capable of mediating the regression of established cancer in a variety of animal tumor models as well as advanced metastatic cancers in humans. We have thus examined the variability of the anti-tumor lytic reactivity of LAK cells obtained from patients with metastatic renal cell cancer (RCC). Tumor cell suspensions were prepared by enzymatic digestion from 37 consecutive renal cell tumors. The mean (+/- SEM) total number of cells recovered was 1.5 +/- 2.2 X 10(9) cells per tumor. The percentage of tumor cells in the suspension was 39.1 +/- 3.3% (range: 6 to 75%). Thirteen of 13 different fresh renal tumor cell preparations tested in 57 experiments and tow of two renal tumor lines tested in 10 experiments were all lysed by LAK cells. RCC patients, like normal donors, generated good LAK effectors with broad antitumor activity against autologous as well as allogenic tumors. Both renal and nonrenal tumors were equally lysed by LAK cells. LAK killing of the erythroleukemic tumor lines K562 and Daudi was significantly better than the lysis of fresh autologous and allogeneic tumor targets or cultured RCC tumor lines. Short term tumor cultures derived from renal cancer preparations proved to be sensitive and reliable tumor targets for studying the in vitro killing by LAK cells. Clinical trials testing the therapeutic role of LAK cells and IL-2 in patients with advanced renal cell cancer are currently in progress.