An antigen-independent physiological activation pathway for L3T4+ T lymphocytes

Interleukin-12/T cell stimulating factor, a cytokine with multiple effects on T helper type 1 (Th1) but not on Th2 cells

At least two subsets of CD4+ T helper cell lymphocytes termed Th1 and Th2 exist in the mouse and probably in humans. They are characterized by the secretion of different lymphokines and by their functional behavior. Dysregulated expansion of one or the other subset may be one reason for the development of certain diseases. Thus, it is of importance to define the signals involved in the differentiation and activation of the two Th cell subsets. It is known and has been confirmed in this report that the cytokine interleukin (IL)-1 acts on Th2 cells but not on Th1 cells. We now report that a previously identified cytokine which was provisionally termed T cell stimulating factor is identical with IL-12 and exhibits a reciprocal behaviour to IL-1. IL-12 has several effects on Th1 cells. It can induce the proliferation of certain Th1 cells in combination with IL-2. Synthesis of interferon (IFN)-gamma by Th1 cells can be triggered by IL-2 plus IL-12. In contrast to the IFN-gamma production observed after T cell receptor (TcR) CD3 stimulation of Th1 cells with lectin Concanavalin A the IFN-gamma production induced by IL-12 + IL-2 is insensitive to the immunosuppressive drug cyclosporin A. Furthermore, IL-12 enhances the TcR/CD3-induced synthesis of IFN-gamma of several Th1 clones. Finally, IL-12 (+IL-2) induces homotypic cell aggregation of Th1 clones. This type of cell aggregation depends on the participation of LFA-1 and ICAM-1 molecules. In all activation systems with Th1 cells no effect of IL-1 was demonstrable. In contrast, only IL-1 but not IL-12 served as a co-stimulatory signal for several Th2 cell lines activated via the TcR/CD3 complex.

Components of an antigen-/T cell receptor-independent pathway of lymphokine production

The general way to induce the synthesis of lymphokines by T cells is the stimulation through the T cell receptor (TcR) complex which results in an increase of intracellular [Ca2+] and in the activation of a tyrosine kinase as well as of protein kinase C. Lymphokine production induced via the TcR is inhibited by the immunosuppressive drug cyclosporin A (CsA). However, an alternative pathway of lymphokine production exists. Several T lymphocyte clones can synthesize interferon-gamma (IFN-gamma), granulocyte-monocyte colony-stimulating factor, and small amounts of interleukin (IL3) when stimulated with syngeneic or allogeneic accessory cells (AC) plus IL2. In contrast to the TcR pathway the alternative pathway does not require a rise of intracellular [Ca2+] and is insensitive to the effects of CsA. In this report we provide evidence for the involvement of T cell-stimulating factor (TSF)--a probably novel murine cytokine--in the alternative pathway of lymphokine production. It is shown that fixation of the AC with carbodiimide or treatment of the AC with UV light greatly reduces their capacity to induce (in combination with IL2) the synthesis of IFN-gamma by T cells. This function is restored by addition of TSF. Moreover, TSF alone (without IL2) in combination with fixed AC can induce the synthesis of substantial amounts of IFN-gamma. Furthermore, TSF in combination with IL2 can stimulate freshly isolated spleen cells to produce IFN-gamma. The target cell resides probably in the non-B cell, non-T cell population.

Requirements for the growth of TH1 lymphocyte clones

Besides the signal generated in a T lymphocyte after triggering the T cell receptor (TcR), most lymphocytes need a "second signal" to become fully activated. The necessity and nature of the "second signal" differs between different types of T cells. At the level of CD4-positive T helper lymphocytes interleukin 1 (IL 1) serves as "second signal" for those of the TH2 subtype (IL4, 5, 6 producer) but not for those of the TH1 subtype (IL 2, IFN-gamma producer). This correlates with the absence of the IL 1 receptor at the surface of TH1 clones. We report herein the further purification of T cell stimulating factor (TSF), a soluble mediator involved in the proliferation of TH1 lymphocytes. A preparation free of detectable IL 1, 2, 4 and IL 6 activity could act as "second signal" required for the growth of TH1 lymphocytes in a TcR-mediated, as well as a TcR-independent activation system. In addition, we suggest that IL 1 can influence the proliferation of TH1 clones in an indirect way, probably via the induction of TSF in accessory cells.

Carrier presentation: a possible strategy for selective in vivo amplification of specific T cells

A strategy for selective in vivo amplification of specific T cells by carrier presentation is outlined here. This hypothesis addresses the possible physiological significance of expression of MHC II molecules by T cells, and attempts to highlight the possibility of harnessing the phenomenon for therapeutic use in medicine.

The processing and presentation of mycobacterial antigens by human monocytes

The processing and presentation of whole irradiated Mycobacterium tuberculosis (Mtb gamma) and its purified protein derivative (PPD) by the peripheral blood monocytes from healthy Bacillus Calmette Guérin (BCG)-vaccinated individuals was investigated. To study processing and presentation as events distinct from T cell recognition and proliferation, monocytes were pulsed with antigens for varying time intervals and fixed. The kinetics of presentation indicate that up to 2 h was required for effective presentation of PPD and 2-4 h for Mtb gamma, and that the ability to activate T cells declined as the time interval for which pulsing occurred was increased, so that responses were abolished by 8-10 h. Prefixed monocytes could not present Mtb gamma and PPD to T cells indicating that processing was an essential requisite. Lysosomotropic agents chloroquine, monensin, and leupeptin inhibited the presentation of these antigens suggesting the role of lysosomes/endosomes in processing. Furthermore, monocytes incubated with optimal concentration of antigens for different lengths of time released determinants which were still antigenic but circumvented the need for any further processing. Addition of nonprimed syngeneic monocytes, both untreated or paraformaldehyde fixed to cells which had been pulsed and fixed, restored the responses even at the later time periods when responses were not detected. This second interaction of the monocyte with T cells was not major histocompatibility complex restricted in that the addition of monocytes from another donor was equally effective.

Involvement of soluble mediator(s) different from interleukin (IL) 1 in the antigen-induced IL 2 receptor expression and proliferation of L3T4+ (CD4+) T lymphocytes

Proliferation of T lymphocytes (T cells) requires the interaction of interleukin 2 (IL 2) with the high affinity form of the IL 2 receptor (IL 2R). IL 2 production as well as IL 2R expression are generally induced simultaneously in T cells by the recognition of specific antigen displayed on the surface of syngeneic antigen-presenting cells. The experiments described herein show that the expression of IL 2R has different requirements than the production of IL 2 (and other lymphokines). Stimulation of antigen-specific L3T4+ T cell lines with antigen-pulsed spleen cells (SC) treated with ultraviolet (UV) light results in efficient IL 2 production but only minimal proliferation due to reduced IL 2R expression, as compared to T cells stimulated by antigen and SC without UV light treatment. The reduced IL 2R expression/proliferation correlates with the absence of a soluble mediator(s) termed T cell-stimulating factor (TSF) in the supernatants of T cells stimulated with antigen-pulsed, UV light-irradiated SC. Addition of TSF of these T cells could at least partially restore the proliferative response or enhance IL 2R expression. Because TSF is present in the supernatant of T cells triggered by antigen and SC but is absent when the latter are UV light treated, we suggest that TSF is a product of metabolically active splenic antigen-presenting cells. The macrophage products interleukin 1 alpha + beta as well as some other cytokines show no TSF activity.