Immunosuppression by Fc region-mismatched anti-T cell antibody treatment

Th1/Th2 cells

A large body of evidence indicates the existence of functionally polarized CD4+ T-cell responses based on their profile of cytokine secretion. Type 1 T helper (Th1) cells produce interferon-gamma, interleukin (IL)-2, and tumour necrosis factor (TNF)-beta, which activate macrophages and are responsible for cell-mediated immunity and phagocyte-dependent protective responses. By contrast, type 2 Th (Th2) cells produce IL-4, IL-5, IL-10, and IL-13, which are responsible for strong antibody production, eosinophil activation, and inhibition of several macrophage functions, thus providing phagocyte-independent protective responses. Th1 cells mainly develop following infections by intracellular bacteria and some viruses, whereas Th2 cells predominate in response to infestations by gastrointestinal nematodes. Polarized Th1 and Th2 cells not only exhibit different functional properties, but also show the preferential expression of some activation markers and distinct transcription factors. Several mechanisms may influence the Th cell differentiation, which include the cytokine profile of "natural immunity" evoked by different offending agents, the nature of the peptide ligand, as well as the activity of some costimulatory molecules and microenvironmentally secreted hormones, in the context of the individual genetic background. In addition to playing different roles in protection, polarized Th1-type and Th2-type responses are also responsible for different types of immunopathological reactions. Th1 cells are involved in the pathogenesis of organ-specific autoimmune disorders, Crohn's disease, Helicobacter pylori-induced peptic ulcer, acute kidney allograft rejection, and unexplained recurrent abortions. In contrast, allergen-specific Th2 responses are responsible for atopic disorders in genetically susceptible individuals. Moreover, Th2 responses against still unknown antigens predominate in Omenn's syndrome, idiopathic pulmonary fibrosis, and progressive systemic sclerosis. Finally, the prevalence of Th2 responses may play some role in a more rapid evolution of human immunodeficiency virus infection to the full-blown disease. The Th1/Th2 paradigm also provides the rationale for the development of new types of vaccines against infectious agents and of novel strategies for the therapy of allergic and autoimmune disorders.

Long-lasting unresponsiveness to polyclonal T cell-binding immunoglobulins

We have shown that mice after a single injection of anti-T cell antibody followed by multiple injections of a second xeno-, allo- or syngeneic anti-T cell antibody differing from the former in species origin developed specific, long-lasting tolerance to the second antibody. To characterize the mechanism of this anti-antibody unresponsiveness and the modalities accompanying the preinjection step, we injected mice with anti-pan T, anti-CD4 or anti-CD8 monoclonal antibodies, followed by multiple injections of polyclonal rabbit anti-mouse thymocyte globulin (RbATG). Our observations indicate that: (i) Depletion of CD4+ cells is the most important factor for tolerance induction to subsequently injected RbATG. Non-depleting mAb were less effective, and antibody-induced CD4 modulation or blockade were inconsequential. (ii) The prevention of anti-antibody responses involves specific B cell tolerance, as shown by suppression of anti-RbATG but not anti-bovine serum albumin (BSA) antibodies after challenge of tolerant mice with RbATG-BSA conjugates. (iii) Suppression of anti-antibody responses involves T cell unresponsiveness rather marginally, as demonstrated by in vitro spleen cell restimulation with RbATG and in vivo antibody response to RbATG-fluorescein isothiocyanate hapten conjugate. (iv) Analysis of isotypes of anti-RbATG antibodies does not suggest alterations in Th1/Th2 tuning as being responsible for this kind of tolerance. (v) Over 150-day survival of fully allogeneic skin grafts (CBA-to-C57BL/6) was observed in mice preinjected with anti pan-T or anti-CD4 mAb followed by RbATG. Mice treated with anti-CD4 mAb alone rejected allografts within 21 days and induced anti-antibodies. Taken together, our experiments suggest B cell tolerance as main mechanism in this type of acquired long-term humoral unresponsiveness to foreign, polyclonal, T cell-binding immunoglobulins.

The TH1/TH2 paradigm in allergy

Recent evidence has been accumulated to suggest that allergen-reactive type 2 helper T cells (Th2) play a triggering role in the activation and/or recruitment of IgE antibody-producing B cells, mast cells and eosinophils, i.e. the cellular triad involved in the allergic inflammation. Interleukin (IL)-4 production by a still unknown cell type (T cell subset, mast cell/basophil?) at the time of antigen presentation to the Th cell is critical for the development of Th2 cells. Other cytokines, such as IL-1 and IL-10, and hormones, such as calcitriol and progesterone, also play a favoring role. In contrast, cytokines such as interferon (IFN-alpha, IFN-gamma, IL-12 and transforming growth factor (TGF)-beta, and hormones, play a negative regulatory role on the development of Th2 cells. However, the mechanisms underlying the preferential activation by environmental allergens of Th2 cells in atopic individuals still remain obscure. Some gene products selectively expressed in Th2 cells or selectively controlling the expression of IL-4 have recently been described. Moreover, cytokines and other gene products that dampen the production of IL-4, as well as the development and/or the function of Th2 cells, have been identified. These findings allow us to suggest that the up-regulation of genes controlling IL-4 expression and/or abnormalities of regulatory mechanisms of Th2 development and/or function may be responsible for Th2 responses against common environmental allergens in atopic people. The new insights in the pathophysiology of T cell responses in atopic diseases provide exciting opportunities for the development of novel immunotherapeutic strategies. They include the induction of nonresponsiveness in allergen-specific Th2 cells by allergen peptides or redirection of allergen-specific Th2 responses by Th1-inducing cytokines, altered peptide ligands, allergens incorporated into recombinant microorganisms or bound to appropriate adjuvants, and plasmid DNA vaccination. In severe atopic patients, the possibility of nonallergen-specific immunotherapeutic regimens designed to target Th2 cells or Th2-dependent effector molecules, such as specific IL-4 transcription factors, IL-4, IL-5 and IgE, may also be suggested.