Casting a smaller net into a bigger donor pool: A single center's experience with the new kidney allocation system.

The new kidney allocation system (KAS) provides additional allocation points for candidates with broad HLA sensitization in an effort to increase transplant rates for this underserved population. Following the implementation of KAS, our center lowered the HLA antibody threshold for listing unacceptable antigens from a cytotoxicity crossmatch level to a flow cytometric crossmatch level increasing Calculated Panel Reactive Antibody (CPRA) values and allocation points, yet restricting acceptable donor HLA phenotypes. As a result, many sensitized candidates were transitioned from 50% to 98% CPRA categories into the 99% CPRA regional share and 100% CPRA national share categories. Exposure to these larger donor pools significantly increased transplantation with compatible donors for 100% CPRA candidates, but regional sharing was not sufficient to increase transplantation rates for our 99% CPRA candidates. Competition within the 100% CPRA cohort identified inequities for 99.99-100.0% CPRA candidates and highlighted the continued need for desensitization therapies to reduce immunological barriers and provide transplant opportunities for the most highly sensitized candidates.

Variable HLA expression on deceased donor lymphocytes: Not all crossmatches are created equal.

Flow cytometric crossmatch tests are used to detect donor-specific antibody and determine eligibility for transplantation. Crossmatch sensitivity is dependent on lymphocyte quality, to include HLA expression on the cell surface. The impact of HLA expression variability on crossmatch reactivity was examined using lymphocytes isolated from different donor types: deceased donor (DD) versus living donor (LD) and tissue sources (blood, spleen, or lymph nodes). HLA class I expression was similar on B cells isolated from LD blood, DD spleen, and DD lymph nodes, but significantly lower on B cells isolated from DD blood (p = 0.0004). In contrast, class II expression on B cells and class I on T cells were significantly higher in LD blood than all DD tissues. Within DD tissues, spleen provided the highest expression of class II on B cells and class I on T cells. HLA expression on B cells, but not T cells, was impacted by memory (CD27+) versus non-memory status. Importantly, HLA expression differences on lymphocytes isolated from the same donor but different tissues impacted crossmatch outcomes. HLA expression is impacted by multiple factors and should be routinely monitored to ensure crossmatch sensitivity and to reconcile crossmatch strength with solid phase HLA antibody analyses.

Impact of pronase on flow cytometric crossmatch outcome.

Pronase treatment of lymphocytes is used to reduce nonspecific binding of immunoglobulins in flow cytometric crossmatch (FCXM) tests and at higher concentrations to remove CD20 from the cell surface. We examined the effect of pronase treatment on human leukocyte antigen (HLA) expression and on FCXM results. Lymphocytes were tested untreated and after treatment with either 2 mg/mL (10 cell donors) or 1 mg/mL (6 cell donors) of pronase. The 2 mg/mL concentration reduced HLA expression in 28 of 30 (93%) cases. The reduction was statistically significant for HLA class I antigens on T cells (33 ± 10%, p = 0.0006), class I on B cells (23 ± 13%, p = 0.012), and class II on B cells (45 ± 37%, p = 0.005). FCXMs were performed using pronase-treated and untreated cells. The 2 mg/mL concentration of pronase reduced reactivity in 5 of 16 (31%) tests of T cells and 15 of 16 (94%) tests of B cells. Of the remaining 11 T-cell tests, the reactivity was unchanged (≤ 10% difference) in 5 and increased by 18-73% in 6. Treatment with 1 mg/mL of pronase significantly increased reactivity in 20 of 23 tests of T cells (87%, p = 6.0 × 10(-5)). These data indicate that pronase treatment may result in erroneous FCXM results.