Lupus Nephritis Exhibiting a Membranoproliferative Pattern in Primary Myelofibrosis: A Diagnostic Challenge of Occult...

Lupus nephritis (LN) with a membranoproliferative pattern is an uncommon but recognized renal manifestation of systemic lupus erythematosus (SLE). While autoimmune myelofibrosis (AMF) has been described in association with SLE, the coexistence of primary myelofibrosis (PMF) with this pattern of LN has not been previously documented. We report a 50-year-old man with JAK2-positive PMF, undifferentiated connective tissue disease (UCTD), and Stage 3b chronic kidney disease who developed nephrotic syndrome and worsening kidney function. Initial serologic evaluation revealed positive antinuclear antibody (ANA), low serum complement C3, and negative antidouble-stranded DNA (anti-dsDNA) antibodies. Kidney biopsy demonstrated a membranoproliferative pattern of injury with mesangial and subendothelial immune complex deposits staining for IgM and C1q. Subsequent testing revealed high-titer anticardiolipin IgM antibodies. The patient met the 2019 EULAR/ACR classification criteria for SLE. Induction therapy with high-dose prednisone and mycophenolate mofetil resulted in a partial remission, with stabilization of renal function on follow-up. To our knowledge, this is the first reported case of LN with a membranoproliferative pattern of injury in a patient with PMF. The case underscores the diagnostic value of kidney biopsy in uncovering occult systemic autoimmune disease in patients with myeloproliferative neoplasms (MPNs), particularly when the clinical and demographic profile is atypical for SLE.