*Purpose: The exact composition of the inflammatory burden during kidney transplant rejection has not been studied on large cohorts. Moreover, the determinants and impact of such composition are unclear. Finally, we’ve recently shown a important heterogeneity in the cellular composition from one patient to another for the same type of rejection.

Our aim was to study the cellular composition of infiltrates in a large cohort of rejection biopsies.

*Methods: We used multiplex immunofluorescence on a cohort of 125 rejections (ABMR n=69; TCMR n=19; Borderline (BL) n=22; mixed n=15) to phenotype T and B lymphocytes, M1 and M2 macrophages as well as NK cells on the same biopsy slide with automated localization (intra- or extravascular) and quantification.

*Results: The total cellular density was maximal during mixed rejection (1242 cells/mm2) significatively superior than ABMR (608/mm2) or BL (611/mm2). During TCMR, the mean cellular density was 892/mm2.

In all rejection types, the majority of cells were located within the interstitium. However, there were more cells in the microcirculation during ABMR.

The 2 main cell types were macrophages, with mostly M2, and T lymphocytes, but there were few NK cells and B lymphocytes.

In the microcirculation, T lymphocytes were the main population. We also confirmed the vast heterogeneity of the cellular composition from one patient to another, within the same type of rejection.

*Conclusions: Multiplex IF applied on a large cohort of 125 rejections confirmed the predominance of macrophages, mainly M2, and T lymphocytes during all types of rejection, with a majority of cells located within the interstitium, and a large heterogeneity from one patient to another.

Determinants and impact of the cellular composition has yet to be determined.

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