Expansion of CD38⁺ Activated B Cells in Circulation as a Manifestation of Early Onset EBV Associated Post-Transplant Lymphoproliferative Disease.

Epstein-Barr virus (EBV)-associated lymphoproliferative disease (LPD) is an uncommon but potentially fatal complication of allogeneic haematopoietic stem cell transplantation (SCT), usually presenting within 6 months post-transplant. Diagnosis of LPD may be difficult as the clinical picture can mimic sepsis or severe GvHD without obvious lymphadenopathy. High blood EBV load is a useful diagnostic feature but in isolation has limited positive predictive value for LPD. We report 2 patients in whom expansion of CD38⁺ activated B cells in peripheral blood was an important diagnostic feature of EBV-associated LPD developing early after T- deplete myeloablative allogeneic SCT. The first patient, a 39 year old female with relapsed acute lymphoblastic leukaemia, presented 4 months after an unrelated donor SCT with a febrile illness in association with EBV and adenovirus reactivation (5.7 x 10⁵ and 2.6 x 10⁶ copies/ml of blood respectively). There was no evidence of lymphadenopathy either clinically or on imaging. Following Cidofovir treatment there was a log reduction in the adenovirus load but EBV load increased coupled with a rising peripheral blood lymphocyte (PBL) count from 0.2 x 10⁹/l to 3.2 x 10⁹/l. The patient died soon after from multi-organ failure. The second patient, a 19 year old female with acute myeloid leukaemia, presented 2 months after a haplo-identical SCT with a similar clinical picture and EBV load of 2 x 10⁷ copies. Her PBL count increased rapidly from 0.5 x 10⁹/l to 11.3 x 10⁹/l. Cell marker studies in both patients revealed B-lymphoproliferation with CD19⁺ B cells constituting 80% of PBL with a profound reduction in NK and T cell numbers. The B cells displayed an abnormal activated phenotype with strong CD38 expression and were CD20⁺, CD22⁺, CD79b⁺, CD5^neg, CD10^neg, and CD103^neg. Concomitant CD23 expression was noted in a proportion of B cells and a minority expressed CD34. Clonal IgH gene rearrangement was noted in the second patient; furthermore, elispot assays demonstrated a complete absence of EBV-specific CD8 T cell immunity. This patient received 2 doses of Rituximab with a log reduction in EBV load, but succumbed to overwhelming sepsis-like syndrome. In both patients, a rising PBL count due to expansion of activated CD38⁺ B cells was an important manifestation of EBV-associated LPD in the absence of obvious lymphadenopathy or organomegaly. This response pattern is in sharp contrast to that seen in immunocompetent patients with infectious mononucleosis (IM) where lymphocytosis is due to proliferation of CD8⁺ T rather than B cells. Interestingly, our recent work on tonsils from IM patients has identified CD38 expression as a useful marker for B cells activated by EBV infection and it is likely that the abnormal B cells noted in the above two patients represent the same. Our experience emphasises the usefulness of peripheral blood cell marker studies in evaluating patients with suspected early onset PTLD following SCT. An expansion of CD38⁺ activated B cells and absence of T cell response can be useful diagnostic features which can aid in making treatment decisions in this difficult situation.