Characterization of human lymphocyte subpopulations: alloreactive cytotoxic T-lymphocyte precursor and effector cells are phenotypically distinct from Leu 2+ suppressor cells.

The OKT8/Leu 2+ human T-cell subset contains cells which perform suppressor and cytotoxic functions. We have recently produced two monoclonal antibodies (termed 2D2 and D12) which define four subpopulations of human E+ cells. Previous studies have shown that the Leu 2+ cells that suppress T-cell proliferative responses have the 2D2+D12+ phenotype. In the present studies, we have used these antibodies and fluorescence-activated cell sorter techniques to characterize the phenotype of cytotoxic T lymphocytes generated in allogeneic mixed lymphocyte cultures. These studies indicate that the cytolytic effector cells which recognize class I major histocompatibility antigens express the 2D2+D12- phenotype. The phenotype of the precursor cells for these cytotoxic T cells was similarly demonstrated to be 2D2+D12-. The subset of E+ cells with NK cytolytic activity expressed the 2D2-D12+ phenotype. These data demonstrate that the Leu 2+ precursor and effector cytotoxic T cells reactive against class I alloantigens are phenotypically distinct from the Leu 2+ cells previously shown to suppress T-cell proliferative responses.

Small, resting B cells can be induced to proliferate by direct signals from activated helper T cells.

We have investigated the role of T lymphocytes in the activation of small, resting B cells. Homogeneous populations of small human B cells were purified with a fluorescence-activated cell sorter by positive and negative immunofluorescent staining techniques. When these small B cells were cultured with mitogen-activated Leu-3+ T cells, significant B cell proliferation was observed. This T cell-dependent B cell proliferative response did not require cross-linkage of B cell surface immunoglobulin molecules or B cell exposure to the mitogens employed for T cell activation. Culture supernatants from activated helper T cells could neither induce small B cell proliferation nor augment the B cell response observed with limiting numbers of T cells. These data indicate that activated helper T cells can provide all the signals required for the activation and proliferation of small, resting B cells and suggest that direct T-B cell contact is a critical element for this T-B cell interaction.

Two phenotypically distinct suppressor T cell subpopulations inhibit the induction of B cell differentiation by phytohemagglutinin.

Human B lymphocytes can be induced to differentiate into antibody-secreting plasma cells by Leu-3+ T lymphocytes stimulated with pokeweed mitogen (PWM), a polyclonal T cell activator. In contrast, other polyclonal T cell mitogens, such as phytohemagglutinin (PHA), also activate Leu-3+ T cells but are relatively ineffective inducers of B cell differentiation. We have performed a series of experiments to investigate the mechanism underlying this apparent paradox. When human B cells were cultured with unfractionated T cells and PWM or PHA, only PWM was able to induce plasma cell formation and immunoglobulin (Ig) secretion. However, when the T cells were treated with mitomycin C (MMC) before culture, both PWM and PHA were able to induce significant B cell differentiation. These data indicated that both mitogens were able to activate the helper T cells required for B lymphocyte differentiation and suggested that MMC-sensitive suppressor T cells were responsible for inhibiting the induction of antibody-secreting cells by MMC-untreated T cells stimulated with PHA. Phenotypic analysis of the T cells capable of suppressing PHA-induced B cell differentiation revealed that small numbers of either Leu-2+ or Leu-3+ T cells could profoundly suppress the B cell differentiation induced by PHA. In contrast, significant suppression of PWM-stimulated B cell differentiation was observed only with relatively large numbers of Leu-2+ T cells. These data confirm previous reports that OKT4+/Leu-3+ T cells can suppress human B cell differentiation and indicate that the difference in B cell differentiation induced by PWM and PHA with MMC-untreated T cells is largely a reflection of the relative potency of these mitogens to activate these phenotypically distinct suppressor T cell subpopulations.

Cross-reactive T-cell antigens among mammalian species.

Fluoresceinated heteroantisera prepared against T cells of rats, monkeys, and humans were reacted withthymus and spleen cells from 11 selected species. These reagents recognized cross-reacting T cell antigen(s) among rodent species (mouse, rat, guinea pig, and hamster) and among primate species (monkey and humans). With one exception, the cross-reactivity was restricted to a phylogenetic order. All three antisera required relatively few absorptions to achieve T cell specificity for related species when compared to absorption requirements for the isologous species. Differentiation antigens within a phylogenetic order thus appear to be more homologous than other cell surface constituents on T cells.