DC-SIGN-mediated infectious synapse formation enhances X4 HIV-1 transmission from dendritic cells to T cells

Abstract

Dendritic cells (DCs) are essential for the early events of human immunodeficiency virus (HIV) infection. Model systems of HIV sexual transmission have shown that DCs expressing the DC-specific C-type lectin DC-SIGN capture and internalize HIV at mucosal surfaces and efficiently transfer HIV to CD4+ T cells in lymph nodes, where viral replication occurs. Upon DC-T cell clustering, internalized HIV accumulates on the DC side at the contact zone (infectious synapse), between DCs and T cells, whereas HIV receptors and coreceptors are enriched on the T cell side. Viral concentration at the infectious synapse may explain, at least in part, why DC transmission of HIV to T cells is so efficient.Here, we have investigated the role of DC-SIGN on primary DCs in X4 HIV-1 capture and transmission using small interfering RNA-expressing lentiviral vectors to specifically knockdown DC-SIGN. We demonstrate that DC-SIGN- DCs internalize X4 HIV-1 as well as DC-SIGN+ DCs, although binding of virions is reduced. Strikingly, DC-SIGN knockdown in DCs selectively impairs infectious synapse formation between DCs and resting CD4+ T cells, but does not prevent the formation of DC-T cells conjugates. Our results demonstrate that DC-SIGN is required downstream from viral capture for the formation of the infectious synapse between DCs and T cells. These findings provide a novel explanation for the role of DC-SIGN in the transfer and enhancement of HIV infection from DCs to T cells, a crucial step for HIV transmission and pathogenesis.

Figure 1.

Generation of DCs knocked down for DC-SIGN expression. (A) DC progenitors were transduced with the indicated lentiviral vectors and differentiated into mature DCs. GFP⁺ DCs were sorted for empty vector-transduced cells (left), whereas GFP⁺ DC-SIGN⁻ cells were sorted for LV-si-SIGN11 (right). Cell percentages corresponding to each quadrant of two-dimensional plots are shown. One representative experiment out of eight is shown. (B) Percentage of DC-SIGN expression is shown for each sorted GFP⁺ cellular population. Means ± SEM of eight independent experiments is shown.

Figure 2.

Analysis of HIV binding and capture by DC-SIGN⁻ DCs or DC-SIGN⁺ DCs. (A) DC-SIGN increases binding of X4 HIV-1 to Raji–DC-SIGN transfectants or immature DCs. Cells were pulsed with HIV (100 ng of p24^gag) for 2 h at 4°C. Cell-bound virus was determined by a p24^gag ELISA. Results are expressed as percentage of p24^gag binding compared with control cells (Raji-LV–DC-SIGN or untransduced DCs). Mean ± SD of three independent experiments is shown. *, Statistically significant differences (Student's t test, P < 0.05). (B) DC-SIGN⁻ DCs (LV-siSIGN 11) capture X4 HIV-1 as efficiently as DC-SIGN⁺ DCs. DCs were incubated with X4 HIV-1 at an MOI of 1 for 2 h at 37°C. Cells were immunostained for surface DC-SIGN and labeled with antibodies against intracellular HIV p24^gag. The percentage of double positive cells for HIV-p24^gag and DC-SIGN was determined on GFP⁺ cells. One representative experiment out of three is presented. (C) Quantification of HIV internalization by DC-SIGN⁺ and DC-SIGN⁻ DCs. Results are expressed as the percentage of p24^gag staining in GFP⁺ DC-SIGN⁺ cells for empty vector-transduced DCs, or in GFP⁺ DC-SIGN⁻ cells for LV-si-SIGN11-transduced DCs. Mean ± SD of three independent experiments is shown. No statistical significant difference was observed between DC-SIGN⁺ DCs and DC-SIGN⁻ DCs (LV-siSIGN 11) (Student's t test, P > 0.05).

Figure 3.

DC-SIGN facilitates transfer of HIV infectivity to target cells in trans. (A and B) DC-SIGN–mediated transfer of HIV to CD4⁺ HeLa-P4-2 cells. Raji transfectants expressing DC-SIGN (A) or immature and mature DCs (B) were transduced with siRNA-expressing lentiviral vectors. DCs or Raji transfectants expressing DC-SIGN were subsequently sorted into GFP⁺ DC-SIGN⁻ (LV-si-SIGN8 and 11) and GFP⁺ DC-SIGN⁺ (empty vector, LV-si-SIGN26). GFP⁺ DC-SIGN⁻ cells (LV-si-SIGN8) were transduced with L-SIGN (LV-L-SIGN) and are GFP⁺ DC-SIGN⁻ L-SIGN⁺ cells. Cells were incubated with X4 HIV-1 at an MOI of 1 at 37°C for 2 h. Infected cells were loaded onto target CD4⁺ HeLa-P4-2 cells, and transfer of HIV infectivity was scored in a single round infection assay. Results are expressed as percentage of the number of infected CD4⁺ HeLa-P4-2 cells compared with control cells (Raji–DC-SIGN [A] or empty vector-transduced DCs [B]). Mean ± SD of three independent experiments is shown. *, Statistically significant differences (Student's t test, P < 0.05).

Figure 4.

DC-SIGN is not required for DC–T cell cluster formation. (A) Sorted mature DCs (GFP⁺) were incubated with highly purified resting CD4⁺ T cells for 30 min at 37°C, allowing DC–T cell cluster formation. Cell nuclei were stained using DAPI (blue). Arrows denote typical DC–T cell conjugates. Representative results of three independent experiments are shown. (B) The number of immunological synapses was determined in each experimental condition by counting on microscope slides. The percentage of DC–T cell clusters formed by LV-si-SIGN11-transduced DCs is shown compared with control DCs (empty vector). Mean ± SD of three independent experiments is shown. (C) Kinetics analysis of DC–T cell cluster formation was performed by flow cytometric analysis. Empty vector- or LV-si-SIGN11-transduced DCs were incubated over time with highly purified resting CD4⁺ T cells. DC-SIGN⁻ DCs (LV-siSIGN 11) form DC–T cells conjugates as efficiently as DC-SIGN⁺ DCs. Mean ± SD of three independent experiments is shown.

Figure 5.

DC-SIGN is present in the infectious synapse between DCs and CD4⁺ T cells. Mature DC-SIGN⁺ DCs were loaded with HIV-GFP for 2 h at 37°C and incubated with highly purified resting CD4⁺ T cells for 30 min at 37°C, allowing infectious synapse formation. Two representative examples are shown (a–c and d–f). DC-SIGN was readily detected at the infectious synapse by confocal microscopy, appearing sometimes enriched (f), but not consistently (c). (green) HIV-GFP; (red) DC-SIGN.

Figure 6.

DC-SIGN promotes infectious synapse formation between DCs and CD4⁺ T cells. (A) Transduced mature DCs were sorted into GFP⁺ DC-SIGN⁻ (LV-si-SIGN11) or GFP⁺ DC-SIGN⁺ (empty vector) cells. Sorted mature DCs were loaded with HIV IN-HA for 2 h at 37°C, and incubated with highly purified resting CD4⁺ T cells for 30 min at 37°C, allowing infectious synapse formation. Representative examples of infectious synapses obtained between DCs transduced with control lentiviral vectors and CD4⁺ resting T cells are shown (a and b). DC-SIGN⁻ DCs (transduced with LV-si-SIGN11) are unable to redistribute internalized HIV from intracellular pools to the infectious synapse (c and d). (a and c) Immunofluorescence microscopy. (b and d) Confocal analysis. (green) GFP-expressing DCs; (red) HIV; (blue) DAPI (nuclei of both DCs and T cells). (B) Kinetics of infectious synapse formation. Quantification over time of infectious synapse formation in DC–T cell immunological conjugates was performed in DC-SIGN⁺ DCs (transduced with empty vector) and in DC-SIGN⁻ DCs (transduced with LV-si-SIGN11). Mean ± SD of three independent experiments is shown.