Baseline characteristics of kidney transplant recipients
The main analysis involved 1,356 patients who underwent kidney transplantation in six French medical centers between 2002 and 2011: 104 in Montpellier, 107 in Paris-Saint-Louis, 188 in Toulouse, 262 in Paris-Necker, 304 in Nancy and 391 in Nantes. The demographics of this study population are listed in Table 1. Most patients were recipients of their first transplant (95%). One hundred and two patients received organs from living donors and 9% of patients received simultaneous kidney–pancreas transplantations. All but two of the relevant covariates for the clinical outcomes analyzed were equally distributed in the MICA-matched and -mismatched patients. There were more retransplantations in the MICA-matched than in the MICA-mismatched groups (10% versus 5%, P = 0.04), and MICA-mismatched transplantations had more HLA mismatches (P < 0.001, P < 0.001 and P = 0.01 for HLA-A, -B and -DRB1 mismatches, respectively; Table 1); both observations are probably due to linkage disequilibrium between MICA and HLA-B.
MICA matching and graft survival
The median follow-up after transplantation was 6.3 years, with a maximum of 12.9 years. The median follow-up was 6.5 and 6.3 years for the MICA-matched and -mismatched patients, respectively. A total of 192 patients (14.2%) had graft failure during follow-up; 1,208 patients (89.1%) survived. Compared with MICA-mismatched patients, MICA-matched patients had a significantly improved graft survival rate (Plog-rank = 0.017), which was the primary endpoint of the study (Fig. 1a). At 5 years after transplantation, graft survival was 96% and 88% for MICA-matched and -mismatched patients, respectively, and this difference in survival rate was also observed when comparing the different mismatching possibilities at the MICA locus (0 versus 1 versus 2 mismatches, Plog-rank = 0.008) (Fig. 1b). The most important impact on graft survival was observed for the case of two mismatches, with rates of 87% and 76% at 5 and 10 years after transplantation, respectively. Based on multivariate Cox regression, MICA mismatching was an independent factor associated with graft loss (HR, 2.12; 95% CI: 1.45–3.11; P < 0.001). Other independent risk factors in the model included age of the donor and recipient, dialysis duration, initial nephropathy, older transplantations, delayed graft function and absence of induction treatment (Table 2). HLA-A, -B and -DRB1 mismatching at a low level of resolution had no impact on graft failure (Extended Data Table 1).
To exclude potential bias due to the difference in the resolution of MICA and HLA genotypes, we analyzed a subset of 862 transplants in which both donor and recipient were retrospectively HLA-typed at second-field resolution, which corresponds to allele-level resolution of MICA typing. Multivariate analysis confirmed the HLA-independent association of MICA mismatches with a higher incidence of graft loss (HR, 1.53; 95% CI: 1.07–2.19; P = 0.018; Extended Data Table 2). Other risk factors for graft loss in the model included age of the donor and recipient, dialysis duration, initial nephropathy, pre-transplantation anti-HLA DSA, number of transplantations, absence of induction treatment, depleting induction treatment and HLA-DQB1 mismatches (Extended Data Table 2). We also confirmed the HLA-B-independent effect of MICA by analyzing HLA-B-matched transplantations in this subset of transplants (n = 33), in which MICA mismatches were still associated with lower graft survival (Plog-rank = 0.015, Extended Data Fig. 1).
Finally, MICA eplet mismatches had a similar association with graft loss, but did not reach statistical significance (Plog-rank = 0.11, Supplementary Fig. 1).
Impact of preformed anti-MICA DSA on graft outcome
Although there is no functional analogy between HLA and MICA molecules, however, to establish whether the observed lower graft survival associated with donor–recipient MICA mismatches might be explained by immunization against MICA (similarly to the situation between HLA mismatches and anti-HLA DSA), we analyzed the pre-transplant sera of 524 patients for the presence of anti-MICA DSA. In this subset of patients, the median follow-up was 5.80 years (with a maximum at 9.58 years) in those with anti-MICA DSA, and 6.04 years (with a maximum at 10.09 years) in those without anti-MICA DSA (Supplementary Table 1). Given that acute rejection is a major cause of kidney transplantation failure (HR, 2.64; 95% CI: 2.15–3.25; P < 0.001, Extended Data Table 3), we assessed whether donor-specific immunization against MICA had a role in this clinical event, which was the secondary endpoint of the study. Acute clinical rejection developed in 77 patients: TCMR in 52 (9.9%) and ABMR in 35 (6.7%), and of those 10 were mixed-type rejections (1.9%). The presence of anti-MICA DSA was found to be an independent risk factor for acute rejection, with a borderline but significant effect on TCMR (HR, 2.11; 95% CI: 1.01–4.42; P = 0.047) and a more important effect on ABMR (HR, 3.79; 95% CI: 1.94–7.39; P < 0.001; Fig. 2a and Table 3). Preformed anti-MICA DSA were not associated with graft loss (HR, 1.32; 95% CI: 0.82–2.10; P = 0.25; Table 3). The association of eplet-specific anti-MICA DSA with ABMR was similar to that of all anti-MICA DSA (Supplementary Fig. 2 and Extended Data Table 4).
One year post-transplant anti-MICA DSA and graft outcome
Immunization against MICA was analyzed using 225 serum samples collected 1 year after transplantation. In this subset of patients the median follow-up was 7.37 years (with a maximum at 9.58 years) and 7.34 years (with a maximum at 9.65 years) in those with and without anti-MICA DSA, respectively (Supplementary Table 2).
Although the presence of anti-MICA DSA at 1 year after transplantation was not associated with a higher incidence of graft failure, it was a risk factor for both TCMR (HR, 1.60; 95% CI: 1.01–2.53; P = 0.043) and ABMR (HR, 9.92; 95% CI: 7.43–13.20; P < 0.001; Fig. 2b and Table 3). Moreover, these associations were maintained when considering only the de novo fraction of these antibodies. Interestingly, the presence of de novo anti-MICA DSA was also a risk factor for graft survival (HR, 1.29; 95% CI: 1.05–1.58; P = 0.014; Table 3). Finally, the presence of anti-MICA DSA after transplantation was associated with a higher frequency of MICA mismatches whether considering all DSA present at 1 year after transplantation (0% versus 24.6% in matched versus mismatched patients, P = 0.0017) or only the de novo fraction of these antibodies (0% versus 13.5% in matched versus mismatched patients, P = 0.05).
We also tested whether specific MICA alleles were more prone to elicit DSA than others. For this purpose, we conducted a chi-squared test for equality of proportions on the proportion of individuals developing de novo anti-MICA DSA conditional on the presence of a specific MICA allele in the donor. There was no specific MICA allele that was associated with a higher rate of de novo anti-MICA DSA (Extended Data Table 5). Finally, when considering only eplet-specific anti-MICA DSA, the association with ABMR was similar to that of all anti-MICA DSA (Supplementary Fig. 3 and Extended Data Table 4).
Synergetic effect of anti-MICA and anti-HLA DSA on ABMR
To evaluate the additive or synergetic impact of anti-MICA and anti-HLA DSA on ABMR, we analyzed the cumulative incidence of ABMR as a function of the presence or the absence of these antibodies before and after transplantation, as determined by single-antigen Luminex assays. The presence of anti-MICA or anti-HLA DSA, before and after transplantation, was a risk factor for ABMR (Fig. 3). In addition, both anti-MICA and anti-HLA DSA had an independent effect on ABMR, before and after transplantation (Extended Data Table 6). Interestingly, the risk of developing ABMR was highest when both types of antibodies were present (HR, 25.68; 95% CI: 3.31–199.41; P = 0.002 for preformed antibodies and HR, 82.67; 95% CI: 33.67–202.97; P < 0.001 for post-transplant antibodies; Fig. 3 and Extended Data Table 6).
Anti-MICA DSA and graft survival in an independent cohort
To further evaluate the role of anti-MICA DSA, we analyzed an independent cohort of 168 patients who had an episode of ABMR with or without anti-HLA DSA between 2013 and 2018. The median follow-up time after biopsy was 4.15 years (with a maximum at 7.90 years) and 4.47 years (with a maximum at 8.18 years) in those without (n = 124) and with (n = 44) anti-MICA DSA, respectively (Supplementary Table 3). The presence of anti-MICA DSA at the time of the diagnostic biopsy was associated with a decreased graft survival rate (HR, 1.71; 95% CI: 1.02–2.86; P = 0.041), as shown by a difference of 19% in survival at 6 years between patients with and without MICA DSA (Extended Data Fig. 2a). Of note, the graft survival was worst when both anti-MICA and anti-HLA DSA antibodies were present, confirming a synergetic effect of these antibodies on graft survival (Extended Data Fig. 2b).