Studying DC-mediated HIV-1 transmission is critical for understanding the mechanisms of cell-cell spread of HIV-1. The interactions between ICAMs and their ligands can facilitate DC-T-cell contact and promote the formation of immunological synapses and antigen presentation (10
). However, the role of ICAMs and their ligands in DC-mediated HIV-1 transmission remains to be clarified. In the present study, we performed functional analyses to examine relative importance of ICAM-1, -2, and -3 in DC-mediated trans
infection of primary CD4+
Although multifactorial interactions between ICAMs and their ligands may facilitate DC-T-cell contact and the formation of immunological synapses, the expression of ICAM-1 on DCs and of LFA-1 on T cells appeared to be critical for DC-mediated HIV-1 trans infection. Blocking of ICAM-1 expressed on DCs significantly decreased both iDC- and mDC-mediated HIV-1 transmission to primary CD4+ T cells and a T-cell line (Fig. ). These data imply that increased ICAM-1 expression could not fully account for enhanced HIV-1 transmission by mDCs relative to iDCs. Furthermore, ectopic expression and antibody blocking suggest that DC-mediated HIV-1 transmission to primary CD4+ T cells might be dispensable for ICAM-2 and ICAM-3. Thus, our results support a model that the interaction between ICAM-1 expressed on DCs and LFA-1 expressed on CD4+ T cells facilitates DC-mediated HIV-1 transmission to primary CD4+ T cells (Fig. ).
FIG. 7. A proposed model for ICAM-ligand interactions in DC-mediated HIV-1 transmission. Our results suggest that ICAM-1-LFA-1 interaction is critical for DC-mediated HIV-1 transmission to CD4+ T cells. However, multifactorial interactions between ICAMs (more ...)
ICAM-1 binding to LFA-1 can enhance T-cell receptor-dependent proliferation of T cells by upregulating various signaling pathways (29
). This activation may contribute to DC-enhanced HIV-1 trans
infection of CD4+
T cells. LFA-1 expression on target cells has been shown to contribute to HIV-1 transmission to CD4+
T cells mediated by DCs (18
) and T cells (22
). By contrast, a recent study suggested that HIV-1 transfer between CD4+
T cells does not require LFA-1 binding to ICAM-1 and is mediated by the interaction of HIV-1 envelope with CD4 (31
). It remains to be examined whether HIV-1 infection of DCs and CD4+
T cells modulates the expression and function of ICAMs and binding ligands.
The ICAM-1-LFA-1 interaction may enhance the formation of virological synapses between HIV-1-associated DCs and CD4+
T cells. A recent study using the lipid bilayers containing ICAM-1 indicated an important role of ICAM-1 in forming virological synapses between CD4+
T cells (40
). We have examined the formation of virological synapses between primary CD4+
T cells and ICAM-1-silenced iDCs and mDCs by fluorescence microscopy. However, no significant difference was observed relative to nonspecific siRNA controls (data not shown). This might be due to the limited sensitivity of the virological synapse assay and low efficiency of ICAM-1 silencing in DCs. Although ICAM-1 silencing could reduce ICAM-1 surface levels on DCs by 40 to 64%, medium to high levels of ICAM-1 remained on the cell surfaces (Fig. ). Further improvement of siRNA knockdown techniques is required to examine the mechanisms by which ICAM-1 silencing inhibits DC-mediated HIV-1 transmission.
Our data suggest that ICAM-2 and ICAM-3 do not significantly contribute to DC-mediated HIV transmission. Interestingly, structural and functional studies of DC-SIGN by Snyder et al. (33
) and Su et al. (36
) indicate that DC-SIGN binds to HIV-1 gp120 more than 100- and 50-fold efficiently than ICAM-2 and ICAM-3, respectively. Moreover, a previous study indicated that replication of X4-tropic HIV-1 is enhanced two- to threefold in ICAM-3-negative Jurkat T cells after 10 dpi (1
), suggesting that ICAM-3 may limit HIV-1 replication even though the mechanism is unknown. However, in our viral infection assays using single-cycle infection and replication-competent HIV-1, no significant difference was observed between ICAM-3-negative and -positive GHOST/R5 cells at 3 dpi (Fig. ). These different observations may result from using different cell lines, HIV-1 strains, and experimental procedures.
Our recent results suggest that intact cytoskeleton is essential for DC-mediated HIV-1 transmission to CD4+
T cells (43
). Altered HIV-1 trafficking and impaired formation of virological synapses primarily accounted for the inhibition of viral transmission by cytoskeletal inhibitors (43
). The actin cytoskeleton contributes to T-cell activation by forming immunological synapses between antigen-presenting cells and T cells (8
). Interestingly, the immunological synapses appear to share structural similarities with the HIV-1 virological synapses and may play a role in viral pathogenesis (11
In summary, our results clarified the role of ICAMs in DC-mediated HIV-1 transmission and provided helpful information in understanding the mechanisms of cell-cell spread of HIV-1. We showed that the interaction of ICAM-1 and LFA-1 plays an important role in DC-mediated HIV-1 transmission to primary CD4+ T cells. Moreover, DC-mediated HIV-1 transmission appears to be independent of ICAM-2 and ICAM-3. Further understanding of HIV-1 and host-cell interactions and the mechanisms of DC-mediated virus transmission will aid in the development of effective strategies to combat HIV-1 infection.