Siglec-1-targeted nanobodies restrict HIV-1 transmission and infection of dendritic cells

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ABSTRACT Human immunodeficiency virus 1 (HIV-1) remains a global health burden affecting over 39 million people worldwide, with approximately one million new infections occurring each year. Sexual transmission remains the predominant route of HIV-1 acquisition, during which the virus crosses mucosal barriers and interacts with host immune cells to facilitate infection. Dendritic cells (DCs) contribute to systemic viral spread and seeding of reservoirs by efficiently capturing HIV-1, leading to infection and subsequent transmission of the virus to CD4 + T cells. Siglec-1 (CD169) is a key receptor involved in HIV-1 capture, and its blockade may help prevent viral transmission. Here, we used newly developed single-domain antibodies, also known as nanobodies, against Siglec-1 as a candidate inhibitor to limit HIV-1 transmission. Using a Siglec-1-overexpressing cell line, we demonstrated that these nanobodies specifically blocked Siglec-1-mediated HIV-1 binding and transmission. Extending these findings to a more physiologically relevant context, the anti-Siglec-1 nanobodies neither induced immune nor cellular activation in DCs, indicating a favorable safety profile for functional applications. Most notably, the anti-Siglec-1 nanobody 2C2 effectively blocked HIV-1 binding as well as infection of DCs. Moreover, both replication-dependent and replication-independent HIV-1 transmission by DCs was also abrogated by the nanobody. Our findings not only underscore the relevance of Siglec-1 in HIV-1 capture but also highlight the therapeutic potential of utilizing host-targeting strategies against infectious diseases. IMPORTANCE Sexual transmission is the main route of human immunodeficiency virus 1 (HIV-1) infection, and novel interventions are needed to prevent this crucial first step. Mucosal dendritic cells play a key role by capturing HIV-1 via attachment receptors, leading to dendritic cell infection and subsequent transmission to T cells, thereby facilitating viral spread and establishment of infection. Here, we show that small, highly specific nanobodies targeting the attachment receptor Siglec-1 strongly interfere with this early stage of HIV-1 transmission. Siglec-1 nanobodies prevented HIV-1 binding to and infection of dendritic cells, thereby blocking transmission to T cells without inducing unwanted immune activation. Together, these findings identify Siglec-1 nanobodies as promising interventions and support the development of host-directed nanobody-based strategies to reduce HIV-1 spread.

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