The human dendritic cell (DC) lineage has recently been unraveled by high-dimensional mapping, revealing the existence of a discrete new population of blood circulating DC precursors (pre-DCs). Whether this new DC population possesses specific functional features as compared to the other blood DC subset upon pathogen encounter remained to be evaluated. A unique feature of pre-DCs among blood DCs is their constitutive expression of the viral adhesion receptor Siglec-1. Here, we show that pre-DCs, but not other blood DC subsets, are susceptible to infection by HIV-1 in a Siglec-1–dependent manner. Siglec-1 mediates pre-DC infection of CCR5- and CXCR4-tropic strains. Infection of pre-DCs is further enhanced in the presence of HIV-2/SIVmac Vpx, indicating that Siglec-1 does not counteract restriction factors such as SAMHD1. Instead, Siglec-1 promotes attachment and fusion of viral particles. HIV-1–infected pre-DCs produce new infectious viral particles that accumulate in intracellular compartments reminiscent of the virus-containing compartment of macrophages. Pre-DC activation by toll-like receptor (TLR) ligands induces an antiviral state that inhibits HIV-1 fusion and infection, but Siglec-1 remains functional and mediates replication-independent transfer of HIV-1 to activated primary T lymphocytes. Altogether, Siglec-1–mediated susceptibility to HIV-1 infection of pre-DCs constitutes a unique functional feature that might represent a preferential relationship of this emerging cell type with viruses.

Human dendritic cell (DC) lineage has been recently unraveled by high dimensional mapping revealing the existence of a discrete new population of blood circulating DC precursor (pre-DC also referred to as AS DC). Among all blood DC subsets, only pre-DC highly express Siglec-1, a lectin-like receptor able to bind HIV-1. We show that pre-DC are uniquely equipped among blood DC populations to promote HIV-1 replication and dissemination. Pre-DC stands out as the most susceptible DC population to infection by both HIV-1 CXCR4-and CCR5-tropic viral particles in a Siglec-1-dependent manner. HIV-1infected pre-DC produce new viral progeny and transmit the virus to CD4 + T cells. Upon TLR activation, pre-DC become resistant to HIV-1 fusion and thus to infection and switch to a replication-independent mechanism of virus transfer to activated primary T lymphocytes mediated by Siglec-1. Thus, beside their role in DC ontogeny, blood pre-DC possess stagespecific properties that HIV-1 may exploit for viral spreading and modulation of the immune response.