NK cell allorecognition shapes reprogramming of neutrophils infiltrating heart allografts

Recent developments in neutrophil biology have demonstrated that neutrophils are phenotypically and functionally heterogeneous. Tissue microenvironments dictate changes in neutrophil cell states in infection and cancer, but little is known about how alloimmune responses or solid organ transplantation influence neutrophil heterogeneity and plasticity.

Here, we used the murine heterotopic heart transplant model in conjunction with high dimensional flow cytometry and transcriptome analysis to interrogate how the alloimmune response and microenvironment of a transplanted organ influence neutrophil subset differentiation and plasticity. Complete MHC mismatched A/J (H2^a) or syngeneic B6 (H2^b) hearts were transplanted to C57BL/6 or B6 background genetically modified recipients.

We uncovered striking differences between neutrophils infiltrating complete MHC mismatched allografts and syngeneic isografts. Bone marrow neutrophil development was highly skewed towards an immature, interferon stimulated gene (ISG)⁺ subset (marked by IFIT1 expression) early after transplant in both allo- and iso-graft recipients. ISG⁺ neutrophils were also the dominant population in the peripheral blood of both recipient groups. In contrast, neutrophils maintained the ISG⁺ phenotype after infiltrating an allograft but appeared to turn off this program upon infiltrating an isograft. The heart graft microenvironment imposed additional reprogramming independent of donor-recipient mismatch, as neutrophils from both allo- and iso-grafts were skewed towards a mature, aged and proangiogenic dcTRAIL-R1⁺ phenotype. Interestingly, while the existing literature indicates that IFIT1-expressing ISG⁺ neutrophils and proangiogenic dcTRAIL-R1⁺ neutrophils are distinct subsets, we identified a novel IFIT1⁺ dcTRAIL-R1⁺ hybrid population that is highly enriched in allografts. Mechanistically, NK cell-mediated innate allorecognition drives this early intra-allograft specific neutrophil phenotypic programing.

These findings provide novel insights into the innate immune allorecognition-mediated regulation of the plasticity of recently described key neutrophil subsets and will enable specific targeting to neutralize detrimental neutrophil subsets and enhance solid organ transplant outcomes.