Lymphocyte stimulation with skin cells. A function of maturation

Functional and phenotypic analysis of isolated human Langerhans cells and indeterminate cells

In the mixed skin cell lymphocyte culture reaction (MSLR), epidermal cells induce the proliferation in vitro of peripheral blood lymphocytes. Using monoclonal antibodies (MCAB) OKT6 specific for Langerhans cells among epidermal cells and an anti HLA-Dr, we analysed which epidermal cells are necessary in MSLR in man. Inhibition and reduction of the proliferative responses in MSLR conducted respectively with 'anti HLA-Dr and complement' and 'OKT6 and complement'-pretreated epidermal cells suggested that not only Langerhans cells but also other HLA-Dr bearing cells play a major role. Immuno-electron microscopic studies of either HLA-Dr (+) or OKT6 (+) enriched and depleted epidermal cells suspensions: (I) were well correlated with results in MSLR; (2) indicated that Langerhans cells express both T6 and HLA-Dr determinants while indeterminate cells express only the HLA-Dr. No other epidermal cells were found to be either OKT6 or HLA-Dr (+). The data strongly suggest that not only Langerhans cells but also OKT6 (-) HLA-Dr (+) indeterminate cells play a major role in lympho-epidermal interactions.

Cellular requirements for the induction of cytotoxic T cells in vitro

The stimulator and accessory cell requirements for the induction of cytotoxic T cell responses to alloantigens in vitro are reviewed. The ability of lymphocytes to stimulate was acquired early in development and was found to be a property of both Ig+ and Ig- cells. The presence of Fc receptors, complement receptors or Ia antigens on lymphocytes did not confer superior stimulator capacity. In contrast to lymphocytes mature macrophages were poor stimulators. When resting lymphocytes were used as a source of stimulator cells there was an additional requirement for an adherent accessory cell population for the induction of cytotoxic T cells. The accessory cell population functioned whether syngeneic or allogeneic with the responder and was required for the induction rather than the maintenance of the response. It was further characterized as an Ig+, theta, FcR+/-, CR +/-, Ia- cell which was not a mature macrophage. Accessory cells could be replaced to some extent by supernatants from spleen cell cultures. A model for T cell induction is proposed where only cells which produce unique inductive stimuli are able to function as stimulator cells. Stimulator cells are defined as those cells capable of stimulating cytotoxic T cell precursors directly and independently. Stimulator capacity is suggested to be a function of activated cells only, thus lymphocytes only stimulate following activation by accessory cells. Cells lacking innate stimulator capacity but exhibiting Ia antigens are suggested to stimulate via an alternative means.

The origin and mechanism of the allograft reaction

Previous explanations for the allograft reaction have been based on the concept that antigen causes immunocyte activation, following engagement of the immunocyte's specific receptor. This notion lead to the concept of immune surveillance, the idea that the evolutionary pressure responsible for the development of the vertebrate immune system involved in allograft rejection was a need to recognize and destroy tumor cells that carried novel antigens. Allografts were rejected because they were, in effect, mistaken for tumor cells. At the practical level, these ideas suggested that a solution to the allograft problem required treatment of the recipient in a way that would reduce or eliminate the recipient's immune response to the grafted tissue. We have rejected these ideas on the grounds that the basic premise, the notion that antigen alone drives T cell differentiation, is invalid. To explain the origin of the allograft response, we have developed a theory of allogeneic reactivity based on the concept that a stimulator cell is required for the activation of blood cells involved in both nonspecific inflammatory reactions and specific cellular immunity. This theory provides a conceptual link between invertebrate and vertebrate alloreactivity and explains why the MHC and factors controlling the expression of T cell activity map in the same region of the genome. According to this theory, it is blood cells carried within the transplanted tissue and not transplantation antigen on the surface of graft parenchymal cells, that constitute the major barrier to allotransplantation. Experimentally we have presented evidence for a two-signal mechanism for T cell activation. Both antigen and an inductive stimulus are required for T cell activation, and neither factor alone induces detectable T cell activation. Organ culture of thyroid tissue for 4 weeks renders it non-immunogenic without altering its antigenic composition. Furthermore, cyclophosphamide pretreatment of the thyroid donor, a procedure that does not destroy the vascular bed of the donor tissue, also reduces its immunogenicity. These findings are of both theoretical and practical importance. They show that transplantation antigen carried on the parenchymal cells of a transplant do not constitute the major barrier to allotransplantation and, at least in the case of thyroid and parathyroid transplantation, indefinite allograft survival can be achieved by treatments directed at the transplanted tissue and not the recipient.