Background:  Monocyte-derived dendritic cells (MDDC) can capture HIV and promote infection in trans of CD4⁺ T cells. DC-SIGN is a C-type lectin expressed on the surface of MDDC that can bind HIV envelope glycoprotein gp120 with high affinity. We aimed to comparatively study the ability of immature and mature MDDC (iMDDC and mMDDC, respectively) to trans-infect HIV-1 to CD4⁺ T cells in vitro, and the specific role of DC-SIGN in such process.

Methods:  Quantification of DC-SIGN antibody molecules bound by MDDC and the B-cell line Raji transfected with DC-SIGN was assessed by flow citometry. Cells were pulsed with single-round infectious pseudotyped HIV-1 containing a luciferase reporter gene and its homologous replicating competent virus to measure viral capture and transmission to susceptible target cells. HIV capture was assayed by p24gag ELISA. Trans-infection ability of MDDC and Raji DC-SIGN was analyzed by luciferase activity or by intracellular staining of p24gag in an in vitro co-culture system with Hut or U-87 CCR5⁺ cell lines. Both viral capture and transmission were followed by electron microscopy. Statistical analysis was preformed using Wilcoxon signed rank test except where indicated.

Results:  mMDDC bound half the number of DC-SIGN antibody molecules per cell that iMDDC did (p <0.0001, paired t-test) and similar amount that Raji DC-SIGN. However, after viral exposition at 37ºC and extensive washing, cell-associated virus was approximately 25 times more abundant in mMDDC than in iMDDC. This observation correlated with electron microscopy analysis. mMDDC had almost 50 times greater ability trans-infecting HIV-1 to target cells than did iMDDC derived from the same seronegative donors (p = 0.03). Therefore, in mMDDC, robust trans-infection does not correlate with DC-SIGN expression levels. Furthermore, employing the MR-1 anti-DC-SIGN monoclonal antibody or mannan, a C-type lectin inhibitor, mMDDC Trans-infection could only be blocked up to a 40% (p = 0.02), whereas in Raji DC-SIGN cell line the same inhibitors could block as much as a 95% (p = 0.001) .

Conclusions:  mMDDC display an enhanced capture and trans-infection ability compared with iMDDC, although this cell type displays fewer DC-SIGN antibody molecules bound per cell. DC-SIGN inhibitors do not efficiently block mMDDC Trans-infection, suggesting that other mechanisms involved in HIV capture besides mannose binding C-type lectin receptors.