Case report: Maternal engraftment of T-cells in a patient with Severe Combined Immunodeficiency due to DCLRE1C pathogenic variant - diagnostic and treatment challenges.

BACKGROUND: Severe combined immunodeficiency (SCID) is a rare but serious congenital condition caused by genetic defects that severely impair the development and function of T cells, B cells, and natural killer (NK) cells. Without early diagnosis and immune system reconstitution, SCID is usually fatal in infancy. The condition requires prompt recognition and hematopoietic stem cell transplantation (HSCT) to restore immune function. This report describes the first genetically confirmed case of SCID in Latvia - caused by a DCLRE1C gene pathogenic variant and diagnosed prior to the implementation of national newborn screening for SCID - complicated by disseminated tuberculosis following Bacillus Calmette-Guérin (BCG) vaccination.

CASE PRESENTATION: The male infant was born after an uneventful pregnancy to healthy parents with no known genetic disorders. The patient experienced recurrent bacterial respiratory infections and received the first scheduled vaccination, the BCG vaccine, at 2 months of age, later than recommended in the national immunization schedule because of parental hesitancy related to concerns about vaccine safety. Shortly after receiving the BCG vaccine, the infant developed erythroderma and a maculopapular rash, followed by pneumonia, acute bronchiolitis, and a bacterial skin infection at three months of age. Immunological evaluation revealed an atypical pattern for SCID, with CD3+ T cells at 53% (abs. 764/μL) and CD4+ T cells at 52% (abs. 748/μL), but a profound deficiency of CD8+ T cells (0.79%, abs. 11/μL). In our case, genetic analysis identified a homozygous pathogenic variant in the DCLRE1C gene. The presence of an XX cell line suggested maternal T-cell engraftment. The patient underwent HSCT; however, post-transplant disseminated tuberculosis required antituberculosis therapy and significantly affected the outcome.

CONCLUSIONS: This case underscores the importance of early diagnosis and, specifically, early antimycobacterial prophylaxis in SCID: although the diagnosis was genetically confirmed at 4 months of age, prophylactic antimycobacterial treatment was not started until disseminated BCG disease was identified 3.5 months later, at 7.5 months of age, and this delay was followed by prolonged multidrug antimycobacterial therapy and a complicated post-transplant course. This highlights the risks of administering live vaccines before immune status is confirmed and reinforces the critical role of newborn screening programs for SCID, which Latvia began implementing on April 1, 2023.