In some reports CD4+ T cells (or CD4+ Treg cells) were also shown to influence the immunodominance of CD8+ T-cell responses, such as during DNA immunization or RSV infection [[38, 39]]. In contrast, the absence of CD4+ T cells did not affect the CD8+ T-cell response hierarchy during influenza virus infection [[40]]. Besides affecting the size of the CD8+ T-cell response, CD4+ T cells have also been implicated

in shaping the phenotypic and functional properties of CD8+ T cells. The absence of CD4+ T-cells during infection with Listeria monocytogenes resulted in impaired effector memory (CD127+ CD62L−) CD8+ T-cell differentiation [[41]] and the absence of CD4+ T cells during LCMV infection prevented the development of central memory (CD44+ CD62L+) find more CD8+ T cells [[42]]. However, whether such phenotypic alterations are

directly inferred by the absence of CD4+ T cells is often unclear, since it should be kept in mind that studying CD8+ T-cell responses in the absence of T-cell help might be problematic in some instances (in particular in the context of replicating infections), where CD4+ T cells might be critically involved in controlling pathogen levels and hence antigen load. It is well known that the level and duration of exposure to antigen critically influences the learn more phenotype and functionality of CD8+ T cells, with longer antigen exposure and higher levels of antigen favoring effector cell differentiation at the expense of memory CD8+ T-cell differentiation [[43, 44]]. In this context it should also be considered that different CD4+ T-cell-deficient models are used to study the requirement of T-cell help, such as CD4– or MHC class II-deficient mice or active depletion of CD4+ T cells using a specific antibody. The caveat of the latter approach is that besides T helper cells, T regulatory (Treg) cells are also depleted and hence it might be difficult to dissect the contributions of classical T-cell help from those of Treg cells in shaping CD8+ T-cell

responses. As mentioned earlier, it is conceivable that PRR ligands of microbial pathogens directly cAMP activate DCs and thereby might compensate for the requirement of T-cell help [[45]]. However, as all viral or bacterial pathogens bear PRR ligands, such as LPS, CpG DNA, dsRNA, ssRNA, lipoproteins, flagellin, etc. that can trigger inflammatory responses and thereby mediate the activation of DCs, it remains unclear which PRR–PAMP (where PAMP is pathogen-associated molecular pattern) interactions render microbial infections T-cell help dependent or independent. There is extensive evidence that infectious agents have developed specific evasion strategies to downregulate inflammation and/or costimulatory molecules, which might be linked to their T-cell help dependence.