Non-adaptive cellular immune responses as studied in euthymic and athymic nude rats. Spontaneous rejection of allogeneic lymphoid cell grafts by natural killer (NK) cells.

The athymic nude (rnu) rat lacks a functional thymus and normal alloreactive T cells. These animals, therefore, have been widely used as tools for studying thymus-independent immune responses. The absence of functional T cells would, to most investigators, indicate that these rats have a defective cellular immune defence. However, although rnu rats accept organ allografts infinitely, they are nevertheless capable of rejecting allografts consisting of lymphocytes or bone marrow cells with increased vigour, and this via antibody-independent mechanisms. These rejection phenomena have operationally been termed allogeneic lymphocyte cytotoxicity (ALC) and allogeneic bone marrow cell cytotoxicity (ABC). Unlike organ allograft immunity this kind of rejection requires no presensitization of the recipient and is surprisingly rapid: it commences within a few hours of i.v. injection of the allogeneic cells and is usually complete by 24 h. Moreover, products coded for by genes within, or closely linked to the major histocompatibility complex (MHC), are clearly involved in the interaction between effector and target cells, as grafted cells from MHC-congenic rat strains are vigorously rejected. In contrast to the defective T cell immune responses in athymic nude rats, the natural killer (NK) cell function is not impaired, and it has been suggested that the spontaneous rejection of MHC-incompatible lymphohematopoietic cells is in fact mediated by NK cells. If the MHC antigens themselves serve as targets in this kind of allorejection, this hypothesis is in apparent contrast with the prevailing view that recognition by NK cells is not guided by, or directed against, MHC-antigens on the target cell surface. Aging athymic nude mice and rats generate cells that rearrange and express T cell receptor (TCR) genes, and this has raised the possibility that T cells or T-like cells in athymic nude animals are responsible for ALC and ABC. This possibility urged the device of an in vitro test system for identification and further characterization of the effector cells in these rejection phenomena. Under appropriate conditions, cells from the rnu rat spleen or liver with natural killer function, i.e. the ability to lyse certain kinds of tumor cells in vitro, are also spontaneously cytotoxic for allogeneic small lymphocytes and bone marrow cells in vitro. Furthermore, these cells can be grown in vitro in the presence of interleukin 2 (IL-2) to generate populations of lymphokine-activated killer (LAK) cells, and these cells have the same spectrum of alloreactivity in ALC and ABC as the native NK cells.(ABSTRACT TRUNCATED AT 250 WORDS)