Donor non-classical monocytes recruit recipient classical monocytes from the spleen by activating donor alveolar macrophages

Primary Graft Dysfunction (PGD) is the predominant cause of early graft loss following lung transplantation. We recently demonstrated that donor pulmonary intravascular non-classical monocytes (NCM) initiate neutrophil recruitment. Simultaneously, host-origin classical monocytes (CM) are mobilized from the spleen and, upon entry into the allograft, permeabilize the vascular endothelium to allow neutrophil extravasation necessary for PGD. Here, we show that CCL2-CCR2 axis is necessary for CM recruitment. Surprisingly, although intravital imaging and multichannel flow cytometry revealed that pharmacological or genetic depletion of donor NCM abrogated CM recruitment, single-cell RNAseq identified donor alveolar macrophages (AM) as predominant CCL2 secretors. Unbiased transcriptomic analysis of human and murine tissues combined with murine knockouts and chimeras indicated that while IL1β was secreted by multiple cell lineages, donor NCM were responsible for the early activation of AM and CCL2 release. IL1β production by NCM was NLRP3 inflammasome-dependent and inhibited by donor treatment using a clinically approved sulphonylurea, Glyburide. Production of CCL2 in the donor AM occurred through IL1R-dependent activation of PKC and NFκB-pathway. Accordingly, we show that IL1β-dependent paracrine interaction between donor NCM and AM leads to recruitment of host AM necessary for PGD. Since depletion of donor NCM, IL1β or IL1R antagonism, and inflammasome inhibition, abrogated recruitment of CM as well as PGD and are feasible using FDA-approved compounds, our findings have potential for immediate clinical translation. Single-cell RNAseq profiles of mouse naïve left lung and allograft after 24hr syngeneic transplantation.