Regulatory T cells in kidney transplant recipients with LIMS1 rs893403 risk genotype

New insights into the pathogenesis of allograft rejection have been provided in the last two decades, caused by additional antigens associated with genetic risk variant of LIM Zinc Finger Domain Containing 1 (LIMS1) gene, and suggest an increased risk of rejection in the allograft recipients with homozygous deletion near LIMS1 ^1,2. Very recently, Sun et al. ³ evaluated donor-recipient mismatches of various single nucleotide polymorphisms (SNPs) of the LIMS1 gene in two established kidney transplant cohorts. Their study of multiple transcriptomic data sets showed a significant role for rs893403 and the new LIMS1 haplotype as cis-expression quantitative trait loci for GCC2 in recipient immune cells which promotes transforming growth factor beta (TGFB1) signaling in lymphocytes, suggesting a regulatory effect in immune cell subsets including FoxP3⁺ regulatory T (Treg) cells.

We previously reported an increased risk of T-cell mediated rejection (TCMR) in patients with rs893403 GG genotype in our cohort of 841 kidney transplant recipients⁴. In the present study, we aimed to examine the distribution of peripheral lymphocyte subpopulations during BK virus DNAemia in transplant recipients with risk genotype (rs893403-GG). This study was approved by the Istanbul School of Medicine Clinical Studies Board (IRB approval number 2011/483–480). All those enrolled in the study provided written informed consent.

In 10 kidney transplant recipients who suffered from BK virus (BKV) DNAemia, we performed flow cytometric studies to analyze peripheral lymphocyte subpopulations at the time of first detection of BKV DNAemia (baseline), and 1 and 3 months after the reduction in immunosuppressive regimens. Two of these patients (20%) had GG genotype while the remaining 8 (80%) had AG. Induction treatment was used in 1 patient with GG and 2 patients with AG genotype (p=0.490). Interleukin-2 receptor blocking antibody, Basiliximab, was used in 1 patient with AG genotype, and the remaining 2 patients were treated with anti-thymocyte globulin. Maintenance immunosuppressive regimens were similar between the groups, a triple regimen consisting of a calcineurin inhibitor, a mycophenolic acid derivative and a low-dose glucocorticoid was used in all patients. Baseline median BKV DNA levels were 5000 (5000–5000) and 5000 (5000–10013) copies/ml in the sera, respectively (p=0.335). Flow cytometry demonstrated that CD4⁺ CD25⁺ FoxP3⁺ Treg cells comprised a higher percentage of total circulating mononuclear cells in patients with GG genotype at the baseline [25.5% (21–30) vs. 16.5% (13–17.5), p=0.036] (Figure 1). Distribution of CD4⁺ T cells, CD8⁺ T cells, and natural killer (NK) cells were similar (p=0.602, p=0.596, and p=0.353). Treg cell fraction gradually decreased after attenuation of immunosuppression due to BKV DNAemia: 18.5% (13–24) vs. 13% (10.5–14.5) at 1 month, and 13% (8–18) vs. 11% (8–15) at 3 months (p=0.290 and p=0.694, respectively).

Figure 1.

Baseline CD4⁺ CD25⁺ FoxP3⁺ regulatory T cells were higher in GG genotype [25.5% (21–30) vs. 16.5% (13–17.5), p=0.036]. Horizontal lines show median values, while error bars represent 25–75 percentiles.

Although all other properties of the patients were somewhat similar, the difference in the distribution of FoxP3⁺ Treg cells we found was interesting. Sun et al. ³ suggested a regulatory effect of the rs893403 and linked SNPs through GCC2 and the TGFB1-SMAD pathway. In our small cohort, we observed an increased fraction of regulatory T cells in the recipients with risk genotype (rs893403-GG).

Although the data in kidney transplantation is lacking, previous studies on hematopoietic cell transplantation (HCT) patients showed that absolute T-cell counts were lower in patients with BK virus reactivation at 3 and 6 months after HCT.^5,6 This was due entirely to delayed recovery of CD4 T cells after transplant. BK DNAemia at 12 months was also negatively associated with T effector CD8⁺CD45RO⁺ and CD4⁺CD45RO⁺ cell counts at 6 and 12 months, respectively. This may be an unappreciated consequence of BK virus reactivation; alternatively, delayed recovery of CD4 T cells may enhance susceptibility to BK virus reactivation. Patients with BK virus disease had lower total CD4 counts for up to 2 years compared with patients who did not experience BK virus reactivation. CD8 T-cell counts in peripheral blood after transplant were similar in patients with and without BK virus reactivation. BKV DNAemia positively correlated with the number of Treg cells after hematopoietic cell transplantation ⁵; similar data for kidney transplantation is lacking, but the higher fraction of Treg subpopulation in our kidney transplant patients with GG genotype was not accompanied by higher BKV DNAemia levels. It is not known whether there might be a functional alteration within these FoxP3⁺ Treg cells or not.

The number of patients is quite small to draw a conclusion from our pilot data. Also, since we started to take samples from the patients at the time of diagnosis of BKV DNAemia, we do not have the data on T cell population before BKV DNAemia. However, our results may suggest a regulatory effect of rs893403-GG genotype on distribution of T-cell subpopulations, particularly FoxP3⁺ Treg cells. Further studies will shed light on the association of allograft rejection, LIMS1 risk genotype rs893403-GG, GCC2 and Treg functions.

Data Availability Statement:

Deidentified data are available upon reasonable request from the corresponding author.