Ongoing Production of Tissue-Resident Macrophages from Hematopoietic Stem Cells in Healthy Adult Macaques

Macrophages are critical orchestrators of tissue immunity, from the initiation and resolution of antimicrobial immune responses to the subsequent repair of damaged tissue. Murine studies with inbred animals housed in a controlled environment have demonstrated that tissue-resident macrophages are comprised of a mixture of yolk sac-derived cells that populate the tissue before birth and hematopoietic-derived replacements that are recruited in adult tissues both at steady-state and in increased numbers in response to tissue damage or infection. While resident macrophages in some murine tissues are readily turned over and replaced, other tissues primarily retain their original, yolk sac-derived complement of macrophages. How this translates to species that are constantly under immunologic challenge, such as humans, is unknown. To understand the ontogeny and longevity of tissue-resident macrophages in nonhuman primates (NHPs), we employed a rhesus macaque model of autologous HSPC transplantation with HSPCs genetically modified to contain unique genetic barcodes and expressing a GFP marker, allowing for subsequent analysis of clonal derivation of leukocyte subsets. We studied the contribution of HSPC to tissue macrophages and their clonotypic profiles relative to leukocyte subsets across tissues and peripheral blood (PB) overt time. We sampled tissue-resident leukocytes from multiple tissues including liver, spleen, lymph node, jejunum and colon, lung (via broncheoalveolar lavage (BAL)) and concurrent PB at several time points post-transplantation. GFP⁺% and the clonal barcodes from lineage cells in tissues and PB were analyzed to assess the degree to which individual myeloid and lymphoid lineage populations had been replaced by output from the transplanted, genetically marked HSPCs in 4 macaques. We also used in vivo bromodeoxyuracil(BrdU) infusions to monitor tissue macrophage turnover in NHPs at steady state.

In all anatomic sites from all rhesus macaque we studied, the frequencies of GFP⁺ macrophages in tissues was as high as in any other population of tissue leukocytes and higher than the frequencies of GFP⁺ T cells, correlating closely with the frequency of GFP⁺ PB monocytes and neutrophils sampled concurrently, indicating tissue-resident macrophages sampled years following transplantation were equivalently derived from transplanted HSPCs as compared to other leukocyte subsets resident in tissues and in PB. Tissue resident macrophages clonal constitution mapped via barcode retrieval was virtually identical to the clonal composition of contemporaneous monocytes in PB, not to short-term myeloid progenitors responsible for initial myeloid engraftment following transplantation, suggested ongoing HSPCs myeloid lineage production contributions to tissue-resident macrophage populations. Moreover, in vivo BrdU labeled provided evidence of tissue macrophage turnover at steady state in otherwise unmanipulated NHPs. These data demonstrate the life span of tissue-resident macrophages can be limited and they can be replenished from HSPCs. Thus, in primates not all yolk-sac derived tissue-macrophages survive for the duration of the host's life. These findings have relevance for viral reservoir dynamics in HIV and gene therapies targeting tissue-resident macrophages.

No relevant conflicts of interest to declare.