A Case of Pneumocystis jirovecii Pneumonia in an Infant with AtaxiaPancytopenia Syndrome

Ataxia-pancytopenia syndrome (ATXPC), a rare autosomal dominant disorder caused by pathogenic variants in the SAMD9L gene, is characterized by variable onset and severity of cerebellar ataxia, hematologic cytopenias, and immunodeficiency of myeloid, B-, and NK-cells. ATXPC has been associated with viral and bacterial infections, but no cases of opportunistic PJP have been reported. At 6 weeks of age, the patient was admitted to the hospital for acute hypoxemic respiratory failure due to rhino/enterovirus bronchiolitis, requiring supplemental oxygen. The increased oxygen requirement later led to intubation. A chest x-ray showed diffuse bilateral opacities concerning for pediatric acute respiratory distress syndrome (PARDS). Bronchoalveolar lavage and silver stain revealed organisms morphologically suggestive of Pneumocystis jirovecii pneumonia (PJP), which was confirmed with a positive serum beta-glucan. Based on the diagnosis of PJP, the patient was started on steroids and Bactrim. Immunology was consulted for further evaluation for possible underlying immunodeficiency. The patient had a normal newborn screening. HIV PCR was negative. Repeated TRECs, CD3+, CD8+, CD19+, and immunoglobulin levels (IgG, IgE) were within normal limits, but modest reduction of CD45 RA/RO ratio and IgA were observed. Additionally, natural killer cell function was low. T-cell proliferation to mitogens were normal, but significantly decreased to candida and tetanus antigens. A significant reduction in the expression of HLA-ABC and HLA-DR was observed in the lymphocytes and monocytes of the patient compared to the control. Interestingly, HLA-DR expression on monocytes was particularly affected. A next-generation sequencing of primary immunodeficiency genes identified likely pathogenic, maternally inherited variants: a heterozygous SAMD9L variant (ataxia-pancytopenia syndrome) and NLRP12 variant (familial cold autoinflammatory syndrome 2). ATXPC has been associated with variable cellular immunodeficiencies. While prior studies have reported recurrent bacterial and viral infections in ATXPC patients, this case is the first to document a PJP opportunistic infection in an infant. It is possible that the NLRP12 variant found in this patient could have also contributed to this clinical presentation. Early diagnosis of ATXPC through clinical recognition and genetic testing may improve prognosis.