Pax-5 Demonstrates Residual B-Cells in Bone Marrow Trephines Post Anti-CD20 Immunotherapy.

Background: Immunohistochemical stains for CD20 are reliable in both formalin and mercuric-based fixed tissues. Anti-CD20 (rituximab (R)) immunotherapy for B- cell malignancies has negated the usefulness of CD20 immunohistochemistry post-therapy. Many other routine B-cell immunohistochemical stains are unreliable in mercuric-based fixatives. Thus, a reliable B-cell stain is needed post-R treatment.

Pax-5 is a transcription factor specific for B-cells that is required for normal B-cell development. It is expressed from the Pro-B stage through to mature B-cells and is expressed on both normal and malignant B-cells. The Pax-5 immunohistochemical stain has proven to be reliable in formalin-fixed tissue. Its role in mercuric-fixed tissue has yet to be determined.

Aim: To demonstrate residual CD20-negative B-cells, using the Pax-5 immunohistochemical stain, on mercuric-fixed bone marrow trephines following R treatment.

Methods: We performed CD20 and Pax-5 immunostains on bone marrow trephines in patients who had received combined chemoimmunotherapy with R. Patients underwent restaging bone marrows midway through treatment (range 2–4 cycles) and at the end of treatment (range 6–8 cycles). We divided analysis into two groups; 1. Patients with overt residual disease on H&E stains. 2. Patients with morphologic complete remission on trephine.

Results: Patient diagnoses included: CLL/SLL n=20; Follicular lymphoma n=2; Mantle cell lymphoma n=1; Waldenstrom macroglobulinaemia n=3. Patients’ morphologic responses were: complete remission (CR) n=5; partial remission (PR) n=9; nodular partial remission (nPR) n=10; no response (NR) n=2. In the non-CR group (PR, nPR, NR) 4 patients were positive for CD20 and all 21 patients were positive for Pax-5. In the CR group 0 patients were positive for CD20 and 5 patients were positive for Pax-5.

Conclusion: Pax-5 is a robust and reliable immunohistochemical stain that demonstrates B-cells post anti-CD20 immunotherapy. While it stains both normal and malignant B-cells, it may prove to be a useful tool in assessing minimal residual disease following immunotherapy.