Anti-HLA antibodies in lung transplantation: diagnostic pitfalls and future directions

Lung transplant recipients face high rejection rates, causing significant disease burden and limiting long-term outcomes. Among immunological factors affecting lung allografts, anti-human leukocyte antigen (HLA) antibodies, particularly donor-specific anti-HLA antibodies (DSAs), are key mediators of antibody-mediated rejection. Yet, the biology, detection and interpretation of DSAs remain incompletely defined across the pre-, peri- and post-transplant continuum. HLAs are highly polymorphic immune recognition molecules, and donor–recipient mismatches drive alloimmune responses. HLA typing is used to assess genetic disparity, but low-resolution approaches risk misclassifying mismatches and DSAs, whereas high-resolution typing improves diagnostic accuracy yet is not universally implemented. Standard HLA antibody monitoring assays report median fluorescence intensity. However, median fluorescence intensity does not convey information on the affinity, avidity or functional capacity of these antibodies, and may be affected by technical factors such as bead saturation, the Hook effect or binding to denatured HLAs. Subclass profiling adds further complexity: IgG1 and IgG3 are potent complement activators, whereas IgG2 and IgG4 have weaker or regulatory roles and differ in clearance by apheresis. Functional assays, including C1q- or C3d-binding HLA antibody detection assays and emerging endothelial or natural killer-cell-based platforms, offer additional insights into antibody-binding, complement activation and cytotoxic potential, but are not routinely applied in current clinical practice. Prospective mechanistic studies are required to define the clinical benefit, cost-effectiveness and optimal integration of these advanced immunological tools into routine practice pre-, peri- and post-lung transplant. DSAs drive lung allograft injury, yet their detection and interpretation remain inconsistent. Implementation of high-resolution HLA typing and functional assays may improve risk assessment and guide future clinical practice.