Conny Hardt

Impact of Thymus on the Generation of Immunocompetence and Diversity of Antigen-Specific MHC-Restricted Cytotoxic T-Lymphocyte Precursors

The thymus represents the primary iymphoid organ producing (thymus derived) lymphocytes. Independent of antigenic stimulation, immunocompetent Tlymphocytes develop within this organ from prethymic precursor cells, which in turn are derived from hemopoetic stem cells. Thus, one ofthe prime roles ofthe thymus is to provide the microenvironment necessary for maturation and differentiation of prethymic T-precursor cells into mature T-cells, which are subsequently exported as post-thymic T-lymphocytes. In addition, production of one (or several) thytnic mediator(s), probably active at distinct sites, appears to be essential for maturation of immunocompetent T-cells (for review see Miller 1979). Not only the thymus, but also gene products of the MHC exert a profound influence on T-cell reactivity. It has been shown that the frequency of alloreactive T-lymphocytes is at least 100 times as high as the frequency of cells reacting to foreign antigens (Simonson 1967, Fischer-Lindahl & Wilson 1977, MacDonald et al. 1980). To account for this high frequency, Jerne (1971) originally proposed that the repertoire of T-cells depends on a set of germ-line vgenes which code for structures essentially complementary to the MHC-alleles of that species. In the thymus, the T-cell precursors that are specific for selfMHC gene products are first selected by self-MHC-antigens encountered on thymic epithehal cells and allowed to differentiate and to proliferate. In a second

The in Vivo Effects of Interleukin 2 (TCGF)

This brief review of our experiments concerning the in vivo activity of crude Il-2 led us to the following conclusion: The first is the existence, in vivo, of a cyclophosphamide-sensitive T-cell controlling the activity of a serum born Il-2 inhibitor in thymus-bearing normal mice. Under in vivo conditions which are characterized by high Il-2 inhibitor activities, locally applied Il-2 administered along with antigen amplified in vivo CTL-responsiveness, yet the effect observed was poor. Crude Il-2 proved to be a potent immuno-enhancing agent in the athymic (nu/nu) mouse, which lacks Il-2 inhibitor activity. It was found that together with antigen administration of Il-2 to nude mice results in the generation of highly reactive T-helper cells, as well as in the generation of alloreactive CTL.

Multiple Signals Required in Cytolytic T Cell Responses

Our present view on the signal requirements for the primary activation of resting Lyt 2+ cytotoxic T lymphocyte precursor (CTL-p) is summarized in Fig. 1. As discussed there is compelling evidence for the existence of two distinct T helper cell subsets. While class II MHC-reactive L3T4+ helper T lymphocytes (HTL) have been known for years, we now find within class I MHC-reactive Lyt 2+ T cells interleukin-2 (IL-2)-producing HTL-p which clonally segregates from CTL-p. Since class I MHC-reactive Lyt 2+ CTL-p and IL-2-producing HTL-p occur in the same order of frequency (f = 1 /500-1 /1000) while the frequency of bifunctional Lyt 2+ T cells is at least 10-fold lower, the L3T4 helper cell independent responsiveness of class I-reactive Lyt 2+ T cells (Sprent et al., 1986) can be explained by T-T cell cooperation within Lyt 2+ T cells. Given the functional heterogeneity of class I MHC-reactive Lyt 2+ T cells, how can the signals required for the activation of a subset thereof, namely that of Lyt 2+ CTL-p, be defined? Since the activation of IL-2-producing Lyt 2 HTL-p is strictly accessory cell dependent, while that of Lyt 2+ CTL-p appears not to be, we studied the lymphokine requirement for the activation of limited numbers of highly purified, resting Lyt 2+ T cells exposed to the mitogen Con A. In this system at least three lymphokines contribute to the induction of cytolytic T cell responses. While Con A falls short in inducing IL-2 receptor expression, the lymphokine “IL-2 receptor-inducing factor” (RIF) conveys sensitivity to IL-2. IL-2 in turn induces clonal growth, although the vast majority of clonally proliferating CTL-p are noncytolytic. However, the lymphokine “cytotoxic T lymphocyte differentiation factor” (CTDF) converts proliferating CTL-p into CTL. Based on these results we conclude that the transition of resting Lyt 2+ CTL-p into CTL can be dissociated into three distinct phases, namely preactivation, clonal growth, and differentiation, which are controlled by RIF, IL-2, and CTDF, respectively.

Frequency-Analysis of Precursors of Cytotoxic T Lymphocytes in Radiation Chimeras: Enumeration of Antigenspecific CTL-P Restricted to Thymic MHC- and Bone Marrow-MHC-Determinants

The mechanisms controlling the acquisition of T cell restriction specificity and immunocompetence are, despite of numerous investigations, not well understood. From studies of the CTL-immune responsiveness in thymus- and bone marrow-grafted chimeric mice, it became apparent, that it is the thymus which is crucial not only for the maturation or T cells, but also for the specificity repertoire of the T cells (1,2). From these data it was suggested, that during intra-thymic maturation both mutational events and positive selection mechanisms influence the repertoire such that only T cells restricted to thymic epithelial cell MHC determinants mature and will be exported to the peripheral lymphoid organs (3,4). However, the demonstration of non-thymic MHC restricted CTL in both chimeric (5–7) as well as conventional mice (8–10) are incompatible with models proposing strictly thymus-dependent selection mechanisms. Allo-MHC restricted T cells were found not only within spleen cells, but also within thymocytes of both chimeric (9) and non-chimeric mice (10). Accordingly, both self- and allo-MHC restricted thymocytes mature to immunocompetent T cells.