Kidneys as a Respite for Hematopoiesis Outside the Reticuloendothelial System: A Rare Case of Extramedullary Hematopoiesis in the Kidneys

Introduction: The increased stress, fibrotic replacement, and overcrowding of cells in the bone marrow in myeloproliferative disorders and myelofibrosis can potentiate hematopoietic stem cells to move outside the bone marrow, typically in the liver or spleen, known as extramedullary hematopoiesis (EMH), and it only rarely occurs in the kidney. This case presents a unique example of a patient evaluated for proteinuria leading to a diagnosis of extramedullary hematopoiesis in the kidney of a patient with a myeloproliferative neoplasm. Case Description: An 81 year-old male with hypertension and myelofibrosis secondary to JAK2-positive essential thrombocythemia treated with momelotinib developed worsening proteinuria. His urine protein to creatinine ratio progressively increased to over 1,000 mg/g within 2 years while his creatinine remained in the normal range. A renal biopsy showed glomerular capillary wall remodeling and a chronic thrombotic microangiopathy-pattern with extramedullary hematopoiesis (Fig. 1). He was started on antiproteinuric therapy with losartan. Discussion: Extramedullary hematopoiesis in the kidney is rare but has been noted in patients with myeloproliferative disorders, particularly with marrow fibrosis. While some cases of EMH can present with radiographically detected kidney or perirenal masses and progressive renal insufficiency, many instances of renal EMH are discovered incidentally, as in this patient who only demonstrated subnephrotic proteinuria, which was adequately explained by hyperfiltration due to adaptive changes from nephron loss and altered glomerular filtration barriers from microvascular injury and repair. This case raises awareness of EMH in patients with primary hematologic neoplasms, as this diagnosis often alters clinical management. It also highlights a link between the renal system and the RES, with the kidney serving as a hematopoietic stem cell niche during times of physiologic stress.