Background: Presence of donor specific anti-HLA antibody (HLA-DSA) has significant impact on the short term and long term allograft outcomes after kidney transplantation. However, the impact of low dose HLA-DSA has not been fully investigated yet. The aim of this study is to investigate the impact of low dose HLA-DSA between living donor transplantation (LDKT) versus deceased donor kidney transplantation (DDKT).
Methods: This study is a retrospective observational study. From January 2010 to December 2018, 627 living donor kidney transplant recipients at Seoul St. Mary’s Hospital and 551 deceased donor kidney transplant recipients at Seoul St. Mary's Hospital and Dongsan Medical Center were enrolled. The low dose HLA-DSA was defined as positive DSAs and negative crossmatch. The primary outcome was the development of biopsy proven acute antibody mediated rejection (BP-ABMR) and the secondary outcomes were change of allograft function, death-censored graft loss rate and patient mortality. 
Results: Of total 627 living donor kidney transplant recipients, 567 patients were negative HLA-DSA group and 60 patients were low dose HLA-DSA group. And total 551 deceased donor kidney transplant recipients, 512 patients were negative HLA-DSA group and 39 patients were low dose HLA-DSA group. In living donor kidney transplantation, acute antibody mediated rejection was increased in the low dose HLA-DSA group than in the negative HLA-DSA group significantly. In univariate analysis, the odds ratio of low dose HLA-DSA was 3.826 (1.695-8.636), and the odds ratios were 2.955 (1.071-8.147) and 6.550 (2.420-17.729) in positive HLA-DSA class I and positive HLA-DSA class II, respectively. In multivariate analysis, the odds ratio of positive HLA-DSA class II in the low dose HLA-DSA group was 8.179 (2.916-22.945) but positive HLA-DSA class I and positive total HLA-DSA were not stastically significant. In deceased donor kidney transplantation, acute antibody mediated rejection tended to increase in the low dose HLA-DSA group but was not statistically significant. Change of allograft function, death-censored graft loss rate and patient mortality were not significantly different between the low dose HLA-DSA group and the negative HLA-DSA group in both living donor kidney transplantation and deceased donor kidney transplantation.
Conclusion: Low dose HLA-DSA is associated with increased acute antibody mediated rejection in living donor kidney transplantation. In low dose HLA-DSA, HLA-DSA class II rather than HLA-DSA class I is considered to play an important role in increased acute antibody mediated rejection. However, the impact of low dose HLA-DSA is unclear in deceased donor kidney transplantation.