Over the last decade is has become clear that recognition of lipid antigens by the immune system plays an important role in the host defense to infectious diseases [38
]. In humans, endogenous and exogenous lipid-antigens are presented to T cells by four different CD1 molecules; CD1a, CD1b and CD1c (group I), and CD1d (group II) [39
]. These four types of CD1 molecules have distinct cellular expression profiles as well as different sub-cellular distributions, indicating a sophisticated system to survey the presence of lipid antigens [39
]. Expression of group I CD1 molecules is mainly confined to professional antigen presenting cells (APCs) such as DCs and Langerhans cells, while CD1d is also expressed in monocytes, macrophages, B cell subsets, and some non-hematopoietic cells [7
]. Noteworthy, CD1d is absent from the surface of naïve T cells but can be expressed upon T cell activation [41
]. Thus, CD1 molecules have an expression pattern that largely overlaps with the host cell range of HIV-1.
In recent years it has become evident, that the two accessory HIV-1 proteins Nef and Vpu inhibit the surface expression of CD1d in a concerted action by intervening with the intracellular trafficking of CD1d. Under normal conditions, CD1d molecules, after initial trafficking to the cell surface, constitutively recycle between the surface and endosomal compartments to survey the endocytic system for the presence of lipid antigens [42
]. Nef tends to retain CD1d in the trans-Golgi network and increases its rate of internalization from the cell surface [36
]. Vpu on the other hand decreases the recycling of CD1d from endosomal compartments to the cell surface, and co-localization of Vpu and CD1d in early endosome antigen 1 (EEA-1) positive structures indicates retention of CD1d in the early endosome [23
]. Putting these observations together, we suggest a model where Nef and Vpu in a synergistic fashion achieve the efficient inhibition of CD1d cell surface expression in productively infected DCs (Fig.
). This model is supported by the finding that virus mutants lacking the expression of either Nef or Vpu still partially down-regulate CD1d whereas a Nef/Vpu double-defective virus completely lacks this activity [23
]. Whereas the detailed molecular mechanisms and structural requirements remain to be elucidated, physical interaction between CD1d and Nef as well as CD1d and Vpu has been detected [23
]. It is, however, unclear if the observed interactions are of direct nature or if other cellular co-factors are involved in complex formation and down-regulation. In addition to the effects of Nef and Vpu, a single report suggested that adding soluble HIV gp120 protein to U937 cells reduced surface CD1d expression on these cells [43
]. The potential mechanisms involved and relationship to the Vpu- and Nef-mediated effects on the CD1d antigen presentation pathway remains to be determined.
Fig. (2) HIV-1 Vpu and Nef interfere with CD1d-mediated antigen presentation.
A) The normal AP-2 dependent trafficking of CD1d
through early and late endosomal compartments for antigen loading and presentation to iNKT cells. B) In HIV-1 infected cells this process (more ...)
CD1d internalization from the cell surface requires interaction of a tyrosine-based motif in its cytoplasmic tail with adaptor protein complex-2 (AP-2) [42
]. Mutation of the known AP-2 binding sites in Nef strongly affected CD1d down-regulation, indicating that Nef enhances CD1d internalization by subverting the regular AP-2 mediated trafficking of CD1d [36
]. Although Vpu contains three potential AP-binding motifs in its cytoplasmic domain, Vpu interaction with AP-2 or other adaptor protein complexes has so far not been demonstrated [45
], making involvement of AP-2 in Vpu-mediated inhibition of CD1d expression unlikely. Down-regulation of CD4 and tetherin involves association of Vpu with the cellular co-factor β-TrCP to mediate ubiquitination and proteasomal degradation, and phosphorylation of Vpu at serine residues 52 and 56 may be important in this process [45
]. The contributions of proteasomal and lysosomal degradation in down-regulation of tetherin from the cell surface are, however, controversially discussed [46
]. Moreover, enhancement of virus particle release by Vpu may occur in the complete absence of tetherin degradation suggesting that the two processes are not related [48
]. However, because CD1d down-regulation from the cell surface does not require Vpu phosphorylation (our unpublished observation), involvement of β-TrCP-mediated ubiquitination and proteasomal degradation is unlikely to be involved in this process. These results collectively prompt the search for cellular co-factors binding Vpu and contributing to the Vpu-mediated retention of CD1d in early endosomal compartments.
The consequences of CD1d down-regulation have been assessed in different experimental systems and comprise early and late events of iNKT cell activation. HIV-1 infected DCs, as well as Vpu-transfected cell lines, loaded with the model lipid antigen α-galactosylceramide have a significantly reduced capacity to induce the formation of an immunological synapse with iNKT cells [23
]. Likewise, the activation of iNKT cells is reduced when either Nef- or Vpu-transfected cell lines or HIV-1 infected DCs are used for stimulation, demonstrating the potency of viral interference with CD1d expression [23
]. Reversion of the phenotype by deleting functional expression of Nef and Vpu fully restores the capacity of DCs to induce iNKT cell activation, indicating that HIV-1 infected DCs are not unable per se
to activate iNKT cells [23
Interference of HIV-1 with the group I CD1 system and T cell responses to antigens presented by these CD1 molecules has not been much studied. The effect of HIV-1 on group I CD1 proteins was described in only a single report, where CD1a down-regulation from the cell surface was observed in DCs infected with recombinant HIV-1 pseudotyped with VSV-G [49
]. In that study, expression of CD1b and CD1c was unaffected, and even down-regulation of CD1d was not observed. Mechanistically, down-regulation of CD1a appeared to be linked to the redistribution of this molecule from the cell surface to LAMP-1 positive compartments, an effect mediated by the Nef protein. The biological consequences of HIV-1 interference with CD1a expression remain unclear and need further investigation.