Matching for 12 HLA Alleles Is Associated with a Significantly Superior Survival Due to a Lower Mortality in Recipients of Unrelated Donor Haematopoietic Cell Transplants for Early but Not Late Stage Leukaemia.
Authors: Shaw B.E. , Mayor N.P. , Little A.-M. , Russell Nigel H. , Pagliuca A. , Ethell M.E. , Morgan G.J. , Madrigal J.A. , Marsh S.G.E. 
Published: October 2019

Abstract
Recipient/donor HLA matching is an important determinant of outcome in transplantation using volunteer unrelated donor (VUD). The degree of matching remains controversial. Some data suggest that disease stage is an important factor to consider when assessing the need for a well-matched donor. We analysed the impact of matching for 12 HLA alleles at six loci (HLA-A, -B, -C, -DRB1, -DQB1, -DPB1) in a cohort of 458 patients receiving VUD transplants for leukaemia (142 CML, 170 AML, 146 ALL). Of the pairs, 84 were matched for 12/12 alleles, 250 and 124 were mismatched for one locus or more than one locus respectively (graft versus host vector). Most single locus mismatches were at DPB1 (218/250, 87%). In multiply mismatched pairs the loci were: DPB1 plus one other locus (81), 2 other loci excluding DPB1 (10) and more than 2 loci (33). Patients receiving 12/12 matched grafts had a significantly better survival than those who had mismatched grafts (7 years: 45% vs 30%, p=0.022) and those matched for 10/10 alleles (p=0.050). The outcome was not significantly different dependent on whether the mismatch was at single or multiple loci, nor whether the single mismatch was at DPB1 compared to any other HLA locus. Disease stage was a significant determinant of outcome, with patients transplanted in early stage disease (defined as CR1 or CP1, N=222) having a superior outcome to those with late stage disease (N=236) (7 year: 42% vs 28%; p=0.002) In patients with early stage disease, the survival was significantly better if 12/12 matched compared to the mismatched group (7 years: 62% vs 36%; p=0.005). Other factors associated with a significantly improved survival in this cohort were: patient age below the median (32 years), patient CMV seronegativity and CML (rather than acute leukaemia). In multivariate analysis, pairs matched for less than 12/12 HLA alleles (HR=1.8; CI 1.0–3.0; p=0.034) and acute leukaemia (HR=1.8; CI 1.2–2.6; p=0.003) had a significantly worse survival. The reason for the inferior outcome was a significantly worse transplant related mortality (TRM) in the mismatched pairs (p=0.006). This was 16%, 32% and 48% at 100 days, 1 and 7 years in the mismatched group, compared to 9%, 13% and 21% in the matched group. While the incidence of acute graft versus host disease (GvHD) overall was not increased in the mismatched group, the incidence of grade III/IV GvHD was higher (12/12 match = 0%, single locus mismatch = 2%, multiple loci=13%; p=0.002) and this was associated with a higher TRM (p=0.002). There was no significant impact of mismatching on chronic GvHD or disease relapse. Unlike these data, in the late disease stage cohort there was no effect of 12/12 matching status on survival (7 years: 25% vs 28%, NS) or TRM. However, there was an increase in GvHD in HLA mismatched pairs (acute, p=0.012; chronic, p=0.015) and the presence of cGvHD was protective against disease relapse (p=0.044). These data suggest that in patients transplanted at an early disease stage, matching for 12 HLA alleles is important to improve survival. In later stage disease the presence of an HLA mismatch may increase the incidence of GvHD and consequently of the graft versus leukaemia effect and hence be tolerated overall.