AN UPDATE ON HUMAN TH1 AND TH2 CELLS

The existence of functionally polarized human T cell responses based o n their profile of cytokine secretion in both the CD4+ T helper (Tn) a nd the CD8+ T cytotoxic cell subset has been established. Human Th1 an d Th2 cells not only produce a different set of cytokines but also exh ibit distinct functional properties and preferential expression of som e activation markers, such as LAG-3 and CD30, respectively. Several fa ctors are involved in the Th cell differentiation into the polarized T h1 or Th2 pathway. They include the cytokine profile of 'natural immun ity' evoked by different offending agents, the nature of the peptide l igand, as well as the activity of some costimulatory molecules and mic roenvironmental ly secreted hormones, in the context of different host genetic backgrounds. Polarized Th1-type and Th2-type responses play d ifferent roles in protection, Th1 being effective in the defense again st intracellular pathogens and Th2 against intestinal nematodes. Moreo ver, they are responsible for different types of immunopathological re actions. Th1 responses predominate in organ-specific autoimmune disord ers, acute allograft rejection, unexplained recurrent abortions, and i n some chronic inflammatory disorders of unknown etiology. In contrast , Th2 responses predominate in Omenn's syndrome, transplantation toler ance, chronic graft versus host disease, systemic sclerosis; moreover allergen-reactive Th2 cells are involved in the triggering of atopic d isorders.