The initial evidence that T helper cells condition

the ability of DCs to prime CD8+ T-cell responses was provided by Bennett et al., [11] showing that priming of ovalbumin-specific CD8+ T cells requires that both CD4+ and CD8+ T-cell subsets recognize their antigen on the same DC (cognate T-cell help). In accordance with this finding, several subsequent studies showed that after in vivo priming with noninfectious agents (such as minor histocompatibility antigens, tumor antigens or protein antigens), CD4+ T-cell help is essential for the stimulation this website of a measurable primary CD8+ T-cell response [[12-14]]. In these settings, T-cell help is thought to mediate the activation of APCs via a mechanism that involves CD40/CD40L interaction between CD4+ T cells and APCs, a process which is referred to as DC “licensing”, Hence, it was believed that, exclusively, immunizations with noninflammatory agents require T-cell help due to a lack of “danger signals,” which in turn would promote activation of DCs and thereby replace the need for T-cell help (Table 1). In Protease Inhibitor Library cell line accordance with the “licensing model,” many pathogenic infections (such as lymphocytic choriomeningitis virus (LCMV), VSV, Ectromelia virus, and HIV) induce strong CD8+ T-cell responses in the absence of T-cell help (Table 1) [[4, 33, 34]], most likely due to their ability to directly activate

APCs via pattern recognition receptors (PRRs) [[35]]. Considering that CD4+ T cells modulate various aspects of the CD8+ T-cell response, this simplified model was challenged by the observation that primary CD8+ T-cell responses to several pathogens such as adenovirus [[21]], influenza virus [[25]], herpes simplex virus (HSV) [[22, 23]], and vaccinia virus [[26, 27]] were compromised in the absence of T-cell

help. These findings raised the question of why certain pathogens differ from others in their ability to generate CD4+ T-cell help-independent CD8+ T-cell selleck kinase inhibitor responses; possible explanations will be provided in the following section “What renders certain infections T-cell help dependent?” However, there are even reports using the same infection model documenting discrepant results on the CD4+ T-cell dependence of CD8+ T-cell responses. For instance, primary CD8+ T-cell responses were shown in some reports to depend on CD4+ T cells during infection with vaccinia virus [[26, 27]], while other reports did not find such dependence [[28, 29]]. When carefully comparing the experimental conditions used in these studies, apparent differences included: (i) the dose of the virus inoculum (with higher doses leading to T-cell help-independent CD8+ T-cell responses), (ii) the use of different vaccinia virus recombinants which might vary in their virulence, and (iii) the concomitant transfer of virus-specific, TCR-transgenic CD8+ T cells, thereby increasing the precursor frequency of the responding CD8+ T cells.