Acute rejection and humoral sensitization in lung transplant recipients

Abstract

Despite the recent introduction of many improved immunosuppressive agents for use in transplantation, acute rejection affects up to 55% of lung transplant recipients within the first year after transplant. Acute lung allograft rejection is defined as perivascular or peribronchiolar mononuclear inflammation. Although histopathologic signs of rejection often resolve with treatment, the frequency and severity of acute rejections represent the most important risk factor for the subsequent development of bronchiolitis obliterans syndrome (BOS), a condition of progressive airflow obstruction that limits survival to only 50% at 5 years after lung transplantation. Recent evidence demonstrates that peribronchiolar mononuclear inflammation (also known as lymphocytic bronchiolitis) or even a single episode of minimal perivascular inflammation significantly increase the risk for BOS. We comprehensively review the clinical presentation, diagnosis, histopathologic features, and mechanisms of acute cellular lung rejection. In addition, we consider emerging evidence that humoral rejection occurs in lung transplantation, characterized by local complement activation or the presence of antibody to donor human leukocyte antigens (HLA). We discuss in detail methods for HLA antibody detection as well as the clinical relevance, the mechanisms, and the pathologic hallmarks of humoral injury. Treatment options for cellular rejection include high-dose methylprednisolone, antithymocyte globulin, or alemtuzumab. Treatment options for humoral rejection include intravenous immunoglobulin, plasmapheresis, or rituximab. A greater mechanistic understanding of cellular and humoral forms of rejection and their role in the pathogenesis of BOS is critical in developing therapies that extend long-term survival after lung transplantation.

Figure 1.

Relative incidence of rejection by time post lung transplant. Depicted are hyperacute rejection, acute rejection (including A-grade typical perivascular cellular rejection and lymphocytic bronchiolitis), and chronic allograft rejection or bronchiolitis obliterans syndrome (BOS).

Figure 2.

Structure of major histocompatibility complex (MHC) molecules. The MHC class I molecules are composed of a heavy α chain and a light β2-microglobulin chain. The α chain is composed of three extracellular domains (α1, α2, and α3), a transmembrane-spanning domain, and a small cytoplasmic domain. The α1 and α2 domains together form a peptide-binding groove presenting peptide to CD8+ T cells. MHC Class II molecules are heterodimers with an α and a β chain. Both chains have two extracellular domains, a transmembrane domain, and a cytoplasmic domain. The α1 and β1 domains together form the peptide-binding groove presenting peptide to CD4+ T cells.

Figure 3.

Pathologic examples of acute lung allograft rejection. (A) Grade A1 acute rejection with rare perivascular lymphocytes (hematoxylin and eosin [H&E] staining; ×40). (B) Grade A2 acute rejection with a prominent perivascular mononuclear infiltrate (H&E staining; ×40). (C) Grade A3 acute rejection with extensive perivascular infiltrate extending into perivascular spaces (H&E staining; ×40). (D) Grade A4 acute rejection with a diffuse mononuclear infiltrate with lung injury (H&E staining; ×40). (E) Grade B2R (high grade) lymphocytic bronchiolitis with dense peribronchiolar mononuclear infiltrate (H&E staining; ×40). (F) Immunofluorescence on frozen lung tissue, demonstrating positive C4d staining in subendothelial space and within alveolar septae (immunofluorescent staining; ×400).

Figure 4.

Flow cytometric antibody screening for measurement of panel reactive antibody (PRA). FlowPRA beads are coated with purified human leukocyte antigens (HLA). After incubation with patient serum, and subsequent staining with fluorescein isothiocyanate (FITC)-labeled anti-human IgG, flowPRA beads were analyzed on a flow cytometer. Beads with antibody binding have greater fluorescence intensity as represented by the rightward channel shift compared with the negative control. A percentage value of PRA is calculated based on the area of peak shifted. This patient demonstrated a PRA of 35% for HLA class I and 0% for HLA class II. The double peak in the positive flow histogram is due to different bead populations emitting fluorescence of different intensity.

Figure 5.

Flow cytometric single antigen (SA) bead assay for detection of a group of anti-HLA class I specific antibodies. Each SA bead is coated with multiple copies of a single recombinant HLA antigen. After incubation with patient serum and subsequent staining with FITC-labeled anti-human IgG, SA beads were analyzed on a flow cytometer. A rightward shift of the beads in the patient sample (right-hand plot), as compared with the negative control (left-hand plot), indicates antibody specificity to the HLA antigens analyzed. This patient has anti-A1, A25, and A26 specific antibodies. For this patient, A1, A25, and A26 represent unacceptable antigens.