Humanized FLT3 mice display enhanced tissue engraftment and support HIV‑1 persistence and rebound

Despite advances in antiretroviral therapy (ART), HIV-1 cure efforts remain hindered by viral reservoirs in long-lived myeloid cells and immune-privileged tissues that are less accessible and therefore unlikely to be assessed in human clinical trials. Consequently, there is a critical need for robust research platforms such as immune cell humanized mice to bridge preclinical and clinical HIV research. However, previously described humanized mouse models have demonstrated incomplete hematopoietic development, particularly showing low levels of NK or myeloid cells. Herein, we present the novel humanized FLT3 mouse model that develops NK cells, myeloid progenitors, monocytes, and both functional conventional (cDCs) and plasmacytoid dendritic cells (pDCs) to support HIV-1 infection. Human cord blood derived CD34 ⁺ hematopoietic stem cells (HSC) were engrafted in the FLT3 (Hu-FLT3) and NSG (Hu-NSG) mouse strains for comparison. Our data showed that while Hu-NSG and Hu-FLT3 mice have comparable human lymphocyte levels, the proportion of myeloid cells (including monocytes, pDCs and cDCs) in Hu-FLT3 mice (16.2 %) was three-fold higher than in Hu-NSG mice (5.6 %) and the proportion of NK cells was six-fold higher (12.8 % and 1.9 %, respectively). Both strains successfully supported HIV-1 infection, maintain viral replication for 17 weeks in untreated mice, and proviral DNA was detectable in peripheral blood, bone marrow and spleen. While ART effectively reduced viral load to undetectable levels in four weeks in both strains, we observed viral rebound after treatment discontinuation within 3 weeks, reaching the same levels of viral load pre-ART and mimicking what is observed in people living with HIV (PLWH).Human immune cells and HIV-1 RNA were higher in tissues of Hu-FLT3 mice compared to Hu-NSG mice, mirroring features reported in human tissue reservoirs. Our findings demonstrated that Hu-FLT3 mice support enhanced development of human innate immune cells in blood and tissues, which are associated with higher levels of HIV-1 replication compared to Hu-NSG mice. This study establishes a novel, robust and accessible in vivo platform to investigate potential HIV cure and persistence-targeting interventions with translational relevance to human therapeutic development thanks to the improved and more complete human immune repertoire in Hu-FLT3.