Lymphoid and Myeloid Proliferations After Chimeric Antigen Receptor (CAR) T-Cell Therapy: The Pathologist's Perspective.

Chimeric antigen receptor (CAR) T-cell infusion has led to improved outcomes in patients with B-lymphoblastic leukemia, B-cell lymphoma, and multiple myeloma. The spectrum of post-CAR T-cell hematolymphoid abnormalities is expanding, although they remain under-recognized. Pathologists play a key role in characterizing hematolymphoid proliferation after CAR T-cell therapy. This review presents clinical and pathologic findings of common hematolymphoid proliferation after CAR T-cell therapy, illustrated by selected cases. A review of the literature is presented in the context of individual cases, and our current understanding of the pathomechanism is discussed. Infused CAR T-cells undergo a series of four phases: distribution, expansion, contraction, and persistence. In the expansion phase, transient peripheral blood lymphocytosis occurs, reaching a peak two weeks post-infusion. Delayed contraction of CAR T-cells may give rise to hemophagocytic lymphohistiocytosis-like syndrome. Immune effector cell-associated enterocolitis presents in the persistence phase, about 3-6 months after infusion. Pathologic findings include a T-cell infiltrate in the intestinal mucosa and changes resembling graft versus host disease (GVHD). This entity requires differentiation from infections and from T-cell neoplasms, including those derived from CAR T-cells. Secondary myeloid malignancies follow the same pathways as therapy-related myeloid neoplasm but present with a shorter median latency. It is essential for pathologists to recognize post-CAR T-cell hematolymphoid proliferation to support clinical decision making in a high-risk patient population.