CD8 + T cells play an important role in controlling pathogenic infections and are therefore key players in the immune response. It has been shown that among other factors CD4 + T cells can shape the magnitude as well as the quality of primary and/or secondary CD8 + T-cell responses. However, due to the complexity and the differences among diverse immunization or infection models, the overall requirement, the time points, as well as the specific mechanism(s) of CD4 + T-cell help may differ substantially. Here, we summarize current knowledge about the differential requirement of CD4 + T-cell help in promoting primary CD8 + T-cell responses as well as establishing functional memory CD8 + T cells in various experimental settings. Keywords: CD8 + T cells r Differentiation r Memory r T-cell help CD8 + T-cell responsesA number of different parameters influence, by virtue of their strength and composition, CD8 + T-cell activation; they subsequently also shape the size and the phenotypical and functional properties of the resultant memory CD8 + T-cell pool. These parameters include antigen-specific T-cell precursor frequencies [1], the strength of the T-cell receptor interaction with peptide-MHC complexes, and the signals provided by co-stimulatory receptors, as well as innate immune system derived inflammatory cytokines [2,3]. Among the factors that modulate the activation of dendritic cells (DCs), the cells that are the main inducers of CD8 + T-cell responses, is the help provided by CD4 + T cells. CD4 + T-cell engagement of DCs promotes the upregulation of certain costimulatory molecules (such as CD80 and CD86) on, as well as the release of pro-inflammatory cytokines such as IL-12 by, DCs. Thus, in many defined experimental settings, T helper cells have been implicated in the expansion and survival of CD8 + T cells during the primary response, and have a key role in establishing long-lived, functionally robust memory CD8 + T-cell responses [4][5][6][7]. The concept of T-cell help for CD8 + T-cell responses was Correspondence: Prof. Annette Oxenius e-mail: oxenius@micro.biol.ethz.ch further supported by the finding that chemokines secreted by activated CD4 + T helper cells can play a key role in the recruitment of naïve antigen-specific CD8 + T cells to antigen-bearing antigen presenting cells (APCs) in secondary lymphoid organs [8] or to sites of infection [9]. Moreover, in some experimental settings CD4 + T cells were proposed to directly interact with CD8 + T cells, thereby promoting their activation and expansion [10]. Hence, the overall contribution, as well as the mechanism, of helper CD4 + T cells in the initiation of primary and secondary CD8 + T-cell responses is more complex than initially believed and appears to strongly depend on the specific biology of the immunization/infection system.

The CD4 and CD8 molecules are involved in T cell differentiation and activation. Nevertheless, efficient thymic maturation of helper T cells has been shown in the absence of the CD4 molecule. These CD4-deficient helper T cells expressed alpha beta-TCR and were able to control Leishmania infections and to mediate Ab class switch. Using mice deficient for the CD8 alpha-chain, we investigated whether a similar cytotoxic T cell population was generated in the absence of the CD8 coreceptor. A CD8-deficient cytotoxic T cell population corresponding to the described CD4-deficient helper T cell population was virtually absent both functionally and physically. These results support the idea that thymic maturation is asymmetrical and strongly biased toward the helper phenotype.