Background:  HIV-1 can bind to the C-type lectin receptor (CLR) dendritic cell (DC)-specific ICAM-3 grabbing nonintegrin (DC-SIGN) on DC and be transferred to CD4⁺ T cells. B cells have intimate contact with T cells in the lymphatics, but have not been shown to express DC-SIGN or transfer HIV to T cells via CLR. We therefore examined B cells for expression of DC-SIGN and transmission of HIV-1 to T cells.

Methods:  Peripheral blood mononuclear cells (PBMC) and purified CD19⁺ B cells stimulated with CD40L (1 ug/mL) and interleukin-4 (IL-4) (1000 U/ml) were stained for co-expression of DC-SIGN and activation markers including CD23 by flow cytometry, and for DC-SIGN RNA by reverse transcriptase-polymerase chain reaction (RT-PCR). For transmission studies, stimulated B cells were left alone or pretreated with 20 ug/mL of anti-DC-SIGN monoclonal antibodies (mAb) and incubated with 10^-4 TCID₅₀ of HIV-1 for 2 hours at 37C, extensively washed and co-cultured with phytohemagglutinin (PHA)-activated autologous T cells or the lymphoblastoid cell line CEMx174, and then assayed for p24 production.

Results:  DC-SIGN was expressed on a subpopulation of CD20⁺ B cells in PBMCs of normal donors (mean ± SE% = 7.6 ± 2%, n = 7), over 70% of whom were CD23⁺. On stimulation, expression of DC-SIGN significantly increased from 7.8 (± 2.4)% to 28.9 (± 4.9)% by 24 hours (n = 20; p = 0.0001), and was predominantly on activated B cells. DC-SIGN expression was confirmed by presence of DC-SIGN mRNA in the B cells. When activated B cells were incubated with HIV-1, virus was evident by electron microscope (EM) in early endosomes, similar to HIV-1 infection of DC. Co-culture of these activated B cells with T cells resulted in high-efficiency HIV-1 replication in the cultures. This was confirmed using CEMx174 cells as targets. Purified, unstimulated B cells did not enhance HIV-1 infection in the co-cultures. Pretreatment of activated B cells with anti-DC-SIGN mAb completely inhibited HIV-1 replication in the co-cultures.

Conclusions:  DC-SIGN is expressed on activated B cells, which leads to enhanced infection of T cells in trans. B cells could therefore become vehicles for HIV-1 infection of T cells during antigen processing that involves signaling by activated-CD40L and IL-4-expressing CD4⁺ T helper cells. This supports a critical role for B cells in the pathogenesis of HIV-1 infection, and suggests new targets for therapeutic strategies.

Keywords: B cells; DC-SIGN; HIV-1