Transient paraproteinemia after allogeneic hematopoietic stem cell transplantation is an underexplored phenomenon associated with graft versus host disease.

The clinical and biological relevance of a paraprotein that newly arises after allogeneic hematopoietic stem cell transplantation (allo-HSCT) in non-myeloma patients is unknown. In this study, the incidence, the course, and the clinical impact of paraproteins found after allo-HSCT were investigated in a cohort of 383 non-myeloma patients. Paraproteinemia after allo-HSCT was more frequent (52/383 patients, 14%) than the reported incidence of monoclonal gammopathy of unknown significance (MGUS) in age-matched healthy subjects and, in contrast to MGUS, did not correlate with age. In most patients (32/52, 62%), the paraprotein appeared transiently within the first year after allo-HSCT with a median duration of 6.0 months. Post-allo-HSCT paraproteinemia was significantly associated with graft versus host disease (GvHD) and correlated with a survival benefit within the first year, but not after five years following allo-HSCT. Importantly, patients with post-allo-HSCT paraproteinemia did not progress into a plasma cell myeloma as observed for MGUS inferring a distinct pathogenic mechanism. Skewing of lymphocyte subpopulations and alterations in cytokine levels in GvHD may explain the expansion of a specific plasma cell subset in non-myeloma patients undergoing allo-HSCT. Our data suggests that paraproteinemia after allo-HSCT is a reactive phenomenon rather than the consequence of clonal plasma cell transformation.