The skin as major immune organ of the human body contains a variety of immune cells including epidermal LCs, DDCs and intraepithelial γδ T cells. It is postulated that VZV-infected T cells transport the virus during the viremic phase to skin epithelial cells which
are subsequently infected by the virus20. It was shown in the SCID-hu mouse model that VZV replication in epithelial cells is associated with expression of a gene product that inhi-bits antiviral IFN-α production in foci of infected cells by interfering with Stat1 activation28. In the surrounding non-infected area increased levels of IFN-α could be detected. However, which immune cells in the skin are targeted by VZV has not been investigated so far.
LCs and DDCs are migratory DCs which are the main DC subtype in the steady-state.
In contrast, inflammatory DCs transiently occur during an ongoing infection. Therefore, skin sections of herpes zoster patients were analyzed for these DC subtypes. Immunofluores-cence microscopy analysis showed the disappearance of epidermal CD1a and Langerin expressing LCs. This can be explained by the emigration of LCs to local lymph nodes where they might transfer the captured antigen to resident DCs. This was recently demonstrated in HSV-1 infected mice suggesting an important role of migratory DCs which emigrate from infected skin to local lymph nodes and transfer captured antigens to resident CD8+ DCs which in turn activate CD8+ T cells94,104. On the other hand, migratory submucosal DCs but not LCs were shown to induce a protective TH1 response in mice after vaginal infection with HSV-2106. In sharp contrast to LCs, a strong infiltration of DCs of myeloid origin expressing CD11c, CD1c, CD1b, CD206 and CD209 in close proximity to virion containing vesicles was observed. Recently, it was demonstrated that monocytes recruited to the dermis during Leishmania infection locally differentiate into “dermal monocyte-derived DCs” and induce protective TH1 responses96. Therefore, it seems likely that the observed infiltrated myeloid-derived DCs in herpes zoster lesions are inflammatory DCs myeloid-derived from monocytes. Inte-restingly, a strong infiltration of CD11c negative pDCs was detected in vesicular lesions of a varicella biopsy107. We did not stain for a specific marker of pDCs, but it is possible that different subtypes of DCs play diverse roles during systemic varicella infection or reactiva-tion during herpes zoster.
Despite the strong infiltration of CD3 positive T cells in herpes zoster skin, detection of the intraepithelial γδ T cells within herpes zoster lesions failed. This might be due to a detection limit of the antibody used. On the other side it is possible that intraepithelial Vδ1+ γδ T cells as innate immune cells emigrated early to local lymph nodes to stimulate other immune cells. A protective role for intraepithelial Vδ1+ γδ T cells in HSV-2 infection was recently demonstrated in γδ T cell depleted mice185.
The percentage of γδ T cells in blood of herpes zoster patients was not significantly altered compared to percentages of healthy control donors. It has to be mentioned, that it is unlikely that Vγ9δ2 T cells the major subset in peripheral blood are influenced during reactivation of VZV. However, it would be interesting to investigate the γδ T cell population during systemic varicella infection where several viremic phases occur. Especially for Vγ9δ2 T cells it has been shown that they acquire the ability to function as professional APCs for naive αβ T cells159. Furthermore, cross-presentation of microbial and tumor antigens by activated Vγ9δ2 γδ T cells to CD8+αβ T cells has been described160.
In the SCID-hu mouse model the role of T cells as transport vehicle for viral spread during VZV pathogenesis was revealed186. Moreover, in this study the vaccine showed de-creased replication in the skin compared to clinical isolates of VZV. However, vaccine strain V-Oka and virulent strains of VZV do not differ in the replication efficiency in human cell lines. This finding of our study was also observed by other research groups and suggests that cutaneous immune cells might be responsible for the observed failure of the vaccine strain to replicate efficiently in the SCID-hu mouse model. Furthermore, this hypothesis is in line with the observed finding that vaccinees do not suffer from the typical VZV rash whereas naive persons infected with clinical isolates of VZV develop disseminated varicella.
Several scenarios are possible which might explain the potency of virulent VZV strains to replicate efficiently in the skin. First, it is possible that only virulent VZV strains possess the capacity to infect cutaneous immune cells and modulate their immune function thereby silencing early antiviral immune responses. Next, it is conceivable that both the vaccine and virulent VZV strains infect efficiently cutaneous immune cells but only virulent VZV strains interfere with their immune function. To test these hypotheses epidermal LCs and DDCs were isolated from human skin and tested for their permissivity to VZV infection and subsequent phenotypic changes. LCs and DDCs isolated from human skin reflecting conve-tional DCs during steady-state conditions were both permissive to VZV infection and no difference in the infection efficiency between both DC types was observed. More impor-tantly, the vaccine strain V-Oka and the clinical isolate JoSt could infect these immunologi-cal important cell types equally well. Interestingly, VZV infection did not induce phenotypic maturation of LCs and DDCs as low cell surface expression of CD83 and CD86 was ob-served. Moreover, a decrease in cell surface expression of CD40 was detected on
VZV-infected LC and DDCs. Thus, both vaccine strain V-Oka and clinical isolate JoSt infect cuta-neous DCs but only weakly modulate their phenotype.