Myeloid chimerism better reflects donor stem cell engraftment than whole-blood chimerism in assessing graft function following allogeneic hematopoietic stem cell transplant (HCT). We describe our experience with 130 patients aged younger than 18 years, treated with allogeneic HCT using bone marrow or PBSC from HLA-matched donors for non-malignant diseases, whose pre-transplant conditioning therapy included alemtuzumab and who were monitored with lineage-specific chimerism after transplant. At 6 years post-transplant, overall survival (OS) was 91.1% and event-free survival (EFS) was 81.5%, with no grade III-IV acute GvHD or chronic GVHD observed. Recipient T-cells did not contribute to graft loss. Mixed T-cell chimerism (MC) did not affect EFS, and there was no connection between T-cell chimerism and myeloid chimerism in patients with MC or graft loss. MC significantly correlated with virus infection; more children with MC were CMV seropositive than those with complete chimerism (CC). Additionally, MC was more common in patients with CMV viramia post-transplant. CD8 T-cell reconstitution was affected by viral reactivation, including CMV, with CD8 T-cell counts higher in the MC group than in the CC group. Mixed T-cell chimerism is due to autologous, virus-specific, predominantly CD8, T-cell expansion, and is protective and not deleterious to the recipient.

Background: Herpes simplex virus (HSV) infection is a significant risk in hematopoietic stem cell transplant (HSCT) recipients, and antiviral resistance poses many clinical challenges. Methods: A retrospective case series of eight pediatric patients with acyclovir‐resistant HSV infection, determined via genotypic sequencing, post allogeneic HSCT between 2012 and 2025. Results: Indications for HSCT included primary immunodeficiency, sickle cell disease, myelodysplastic syndrome (MDS), and relapsed/refractory leukemia. All patients received acyclovir prophylaxis. TK mutations were most common ( n = 7, 87.5%). Infections manifested as mucocutaneous disease, pneumonitis, keratitis, enterocolitis, and blood viremia. A total of 87.5% of patients ( n = 7) were treated with second‐line antivirals (foscarnet and/or cidofovir), and three patients were treated with third‐line antivirals (pritelivir). Treatment was complicated by nephrotoxicity ( n = 3, 37.5%). Adjunct treatments included intravenous immunoglobulin ( n = 3, 37.5%) and donor lymphocyte infusion ( n = 1, 12.5%). Poor outcomes were seen across the cohort, including death ( n = 3, 37.5%). T‐cell depletion was used in 50% of patients ( n = 4). Concomitant immunosuppressive therapy was used in 100% of patients ( n = 8) and high‐dose steroids were used in 50% of patients ( n = 4). Graft versus host disease occurred in four patients (50%). Secondary complications included transplant‐associated thrombotic microangiopathy ( n = 3, 37.5%), idiopathic pulmonary syndrome ( n = 3, 37.5%), and graft failure ( n  = 1, 12.5%). Conclusion: Acyclovir‐resistant HSV infection in pediatric HSCT recipients is associated with high morbidity and mortality, limited therapeutic options, and significant treatment‐related nephrotoxicity. Early recognition, early resistance testing, prompt initiation of second‐line therapy, and weaning of immunosuppression are critical. Emerging therapies, such as helicase‐primase inhibitors and adoptive T‐cell therapy, hold promise but remain limited by access and pediatric data.

Our case highlights timely diagnosis and management of pulmonary embolism (PE) and superior vena cava syndrome in a child with a background of Chediak-Higashi syndrome who underwent a bone marrow transplant, the probable precipitant being concomitant autoimmune haemolytic anaemia. He was successfully managed in paediatric critical care with a catheter-directed tissue-type Plasminogen activator (tPA) thrombolysis without any complications. He is currently 3 years post transplant with good immune reconstitution.

Background: The use of romiplostim, a thrombopoietin agonist, has increased in the last decade for the treatment of immune mediated thrombocytopenia and severe aplastic anemia. Its utility has been explored in the management of delayed platelet engraftment and secondary platelet failure during stem cell transplant (SCT), but its use has remained largely anecdotal in pediatric allogeneic SCT. Methods: In this single centre, retrospective study we report the largest pediatric SCT cohort use of romiplostim. Results: Romiplostim was used in 17 children for several indications, principally including poor graft function (PGF) and immune‐mediated cytopenia (IMC), including multi‐lineage cytopenia. The overall response rate (ORR) was 76.5% and the median time to achieve OR was 42 days. No toxicity was observed with romiplostim including marrow fibrosis, clonal evolution and thrombosis with a median follow‐up of 18 months. Romiplostim averted the need for second allogeneic SCT in two patients with late graft failure and the need for stem cell boost (SCB) in three patients. Conclusion: We propose that romiplostim can be safely used in the cytopenia in pediatric SCT to good effect.

RAS-associated lymphoproliferative disorder (RALD), caused by acquired gain-of-function mutations in Kirsten rat sarcoma KRAS or Neuroblastoma rat sarcoma NRAS, is an orphan disease with very few cases reported worldwide. The most common presenting feature is autoimmune cytopenia, which is generally documented to be responsive to immunosuppression. We describe two cases of RALD in which associated autoimmunity proved refractory to multiagent immunosuppression. We also share our experience of haematopoietic stem cell transplantation (HSCT) in the two patients with recalcitrant autoimmunity, adding valuable insight into the distinctive complications and transplant course in this rare disorder. Furthermore, we review the published literature on HSCT in RALD, which includes three other patients, primarily transplanted for malignant transformation of RALD. In addition, we report our experience with targeted therapy, specifically the MEK inhibitor, in this disease and reflect on its role within the treatment paradigm of RALD.