Macrophage Function in Kidney Repair

Repair of the kidney tubules after an acute insult requires the clearance of dead cells and proliferative
repopulation of the tubules by those cells that survive the initial insult. Our data obtained by tracking individual
tubules using high resolution multiphoton Z-stack images on days 1 and 3 after ischemia-reperfusion injury
confirms that the S3 segment of the proximal tubule is the site of most tubular cell loss and intraluminal cast
formation, and that epithelial proliferation and cast clearance dominate the histologic findings in the outer stripe
of the renal medulla from days 2-5 after injury. During this exact time, macrophages (termed mononuclear
phagocytes (MNPs) for uniformity) progressively accumulate in the injured kidney and are specifically induced
to express reparative genes beginning on day 2-3 after ischemia reperfusion injury (IRI). We now show that one
of those pro-repair markers, arginase 1 (Arg1), is selectively induced only in MNPs in the outer stripe directly
adjacent to the injured S3 segment. General depletion of reparative MNPs using liposomal clodronate or
selective knock-out of MNP Arg1 expression using LysM-Cre;Arg1fl/fl mice reduces both proliferative S3 repair
and cast clearance by the injured kidney, with increased mortality, higher BUN and creatinine, and worse tubule
repair in the MNP Arg1 null mice. Single cell RNA sequencing performed on cells isolated on day 3 after IRI or
sham operation demonstrates that these Arg1+ cells make up a distinct subset of MNP that selectively express
the homing receptors Ccr2 and CD74, while injured PT cells express the cognate ligands Ccl2 and Mif. The
specific induction of Arg1 in the OS MNP, combined with the ability of these cells to promote proliferative tubule
repair and cast clearance, has led us to hypothesize that the injured OS forms a niche for recruiting and
selectively activating MNP to facilitate S3 repair.
In this proposal we will use an in vitro OS mimetic to define the critical cellular components and secreted factors
that promote MNP homing and reparative activation in the OS, identify a pharmacologic approach to recreate
this reparative activation, and then use this pharmacologic activation of reparative MNP as well as imaging mass
cytometry and immunofluorescence (IF) analysis to validate these in vitro findings in the mouse kidney after IRI
(SA 1). We will then pursue the mechanism by which reparative macrophages induce epithelial proliferation and
cast clearance using a combination of candidate factor screening, scRNA-seq and proteomic analysis to identify
the MNP-secreted factors that stimulate tubule cell proliferation, as well as the role of reparative MNP in
detoxifying metabolites of PT cell amino acid catabolism and promoting cast degradation at the S3-thin
descending limb junction (SA 2). The combined output of this proposal will provide mechanistic understanding
of how MNPs promote both reparative proliferation and cast clearance, and we will use this information to
develop pharmacologic approaches to mimic these responses and facilitate kidney tubule repair.

Grant Number: 5R01DK093771-10
NIH Institute/Center: NIH
Principal Investigator: LLOYD CANTLEY