# ACK2 antibody conditioning enhances adoptive transfer of hematopoietic progenitors to study central trained immunity in mice

## TL;DR

A new antibody-based method improves the transfer of blood stem cells in mice, helping study how these cells develop and respond to infections.

## Contribution

A non-inflammatory ACK2 antibody conditioning method is introduced to enhance HSPC adoptive transfer in mice.

## Key findings

- ACK2 transiently depletes HSPCs and improves donor engraftment in the bone marrow.
- Neutrophils from C. albicans-exposed HSPCs show enhanced recruitment during secondary infection.
- ACK2 conditioning enables transient engraftment for studying HSPC biology and trained immunity.

## Abstract

Hematopoietic stem and progenitor cell (HSPC) transplantation is a cornerstone for studying hematopoiesis. However, classical conditioning regimens such as irradiation or chemotherapy induce strong inflammation, alter the bone marrow (BM) microenvironment, and severely limit the interpretation of differentiation processes. Moreover, donor HSPC engraftment efficiency in immunocompetent recipients without conditioning is usually very low. In this work, we produced and purified the monoclonal anti-c-Kit antibody ACK2 and tested its capacity to transiently deplete HSPCs in immunocompetent C57BL/6 mice. We defined the in vivo clearance kinetics of the ACK2 antibody from serum, identified the optimal transplantation window, and evaluated donor engraftment efficiency. Intraperitoneal injection of ACK2 induced transient HSPC depletion in the BM, with maximal depletion and complete clearance of circulating antibody at day 4 post-injection. Transplantation of donor HSPCs in ACK2-conditioned recipients at this time point resulted in significantly improved engraftment compared to PBS-treated recipients, particularly in the BM. As a proof of concept, we applied this mouse model to investigate properties of innate immune memory in HSPCs exposed to Candida albicans in vivo. For this, we adoptively transferred HSPCs from infected mice with a non-virulent C. albicans strain and assessed the functional properties of their derived neutrophils in vivo. We found that neutrophils derived from C. albicans-exposed HSPCs displayed an enhanced recruitment to the peritoneal cavity during a secondary C. albicans infection compared to control HSPC-derived neutrophils. In conclusion, here we describe a non-inflammatory, antibody-based conditioning method that enhances adoptive transfer of HSPCs in immunocompetent mice. Consistent with previous reports, ACK2-based conditioning alone does not enable permanent hematopoietic engraftment, but rather facilitates transient donor cell engraftment which provides a versatile methodological tool to study the biology and functional programming of exogenous HSPCs in vivo, including their contribution to trained immunity.