Further diagnostic evaluation excluded maternal T cell engraftment by short tandem repeat analysis. Chromosomal microarray was normal. Next generation sequencing for genetic causes of SCID was performed at 6 weeks of age, identifying a novel TBX1 heterozygous pathogenic variant (c.1176_1195dup20, p.Glu399Glyfs), leading to a frameshift that determined a premature stop codon at position 467. This C-terminal truncation in exon 9 is predicted to disrupt the transactivation domain of TBX1 where the nuclear localization signal is located and lead to loss of normal protein function. TBX1 haploinsufficiency was consistent with T-B+NK+ immunophenotype and facial dysmporphism. Single variant sanger sequencing of both the mother and father showed normal sequence of TBX1. Cultured peripheral blood CD34+ hematopoietic stem cells from the patient differentiated normally up to the CD3+ T cell stage, with pro-T, double-positive and double-negative T cells present as normal levels, indicating that the primary disorder was most likely outside of the hematopoietic compartment. HLA typing of family members identified an HLA-matched six year old sibling who had received all age appropriate vaccines. Other evaluations included a chest X-ray that was remarkable for the absence of thymic tissue, a normal transthoracic echocardiogram, and normal parathyroid hormone and calcium levels.
Given the diagnosis for TBX1 haploinsufficiency, definitive therapy with thymic transplantation was pursued. During this time, the patient remained in protective isolation on antimicrobial prophylaxis. At 2 months of age, she developed a mild maculopapular erythematous rash on her face, trunk, and extremities. Skin biopsy showed spongiotic dermatitis on hematoxylin and eosin staining consistent with eczema. Infectious evaluation for systemic viral infections including EBV, CMV, herpes simplex virus, human herpesvirus 6, and adenovirus (ADV) were negative. Peripheral blood T-cell receptor (TCR) spectratyping at 2 months age revealed an abnormal TCR Vbeta repertoire with 21/28 TCR Vbeta families and sub-families showing an oligoclonal (<5 independent peaks) distribution, two families displaying no peaks, and five families demonstrating a polyclonal, non-Gaussian distribution. She responded well to topical steroids, and the rash quickly resolved. Two months later (4 months of age), she developed fever and rhinorrhea secondary to Rhinovirus. Concurrently, she developed an eczematous generalized rash with associated erythroderma, severe pruritus and alopecia universalis. Laboratory assessment was notable for an elevated IgE, increased absolute eosinophil counts, and CD3+, CD4+, and CD8+ lymphocytosis, mainly with a memory phenotype ( Table 1 ). Repeat peripheral blood TCR spectratyping was consistent with previous abnormal TCR Vbeta repertoire findings. Clinical and laboratory features were consistent with OS. She was treated with high dose systemic steroids and then transitioned to cyclosporine leading to skin rash and alopecia resolution after 3 weeks of treatment. The development of OS led to re-evaluation of treatment strategies and consideration of MSD HCT. However, given the thymic epithelial defect present, a decision to hold and wait for thymic transplantation was made.
At seven months of age, the patient developed new onset protracted vomiting and fever. Infectious evaluation was remarkable for severe adenoviremia (ADV PCR: >1,000,000 copies/mL) and elevated transaminases. Viral load and transaminases continued to increase over the following month despite treatment with cidofovir. Due to persistent adenoviremia despite maximal antiviral therapy, presence of life-threatening end-organ involvement, and lack of expected immune recovery, the patient was subsequently treated with two infusions (2x107cells/m2) of CTLs specific for adenovirus (HLA class-II mediated antiviral restriction). These virus specific CTLs were generated from third-party, healthy donors who were partially HLA-matched (5/10 and 4/10) with the recipient, and whose cells were confirmed to have class II HLA-restrictions that were shared with the patient. Partial clinical improvement with reduction in liver function tests, improvement of liver synthetic dysfunction, and reduced adenovirus viral load were achieved but not sustained ( Figure 2 ). Due to persistent adenoviremia and hepatitis despite antiviral therapy and adenovirus-specific CTL infusions, the patient underwent an unconditioned, unmanipulated bone marrow transplant from 10/10 MSD (10x106 CD34+ cells/kg; 5.2x107 CD3+ cells/kg) 2 months after developing adenoviremia. Immediately after cell infusion, there was a surge of adenoviral load attributed to massive lysis of adenovirus infected cells, that corresponded to development of fulminant hepatitis and ultimately hepatic failure. The degree of viremia ( Figure 2 ) progressively declined and was <1,000 copies/mL within seven weeks post-HCT with associated improvement in transaminases. GvHD prophylaxis included cyclosporine, replaced by tacrolimus on Day +6. Unfortunately, despite a decline in adenoviremia, she developed direct hyperbilirubinemia with biopsy confirmed acute stage 3 liver GvHD ( Figure 3 ) without signs of skin or intestinal GvHD. Despite initial response to systemic corticosteroids and calcineurin inhibitor, chronic liver GvHD developed during withdrawal of immune suppression, ultimately requiring sirolimus (added on Day +79), a course of rituximab (4 doses spaced weekly), ruxolitinib, and extracorporeal photopheresis (ECP) for eight months to achieve adequate response.
Over the last four years, immune reconstitution, and donor chimerism have been monitored closely. She has shown consistently appropriate T cell engraftment with > 90% donor-derived T cells ( Figure 4 ). After an initial robust T cell response, CD3+, CD4+, and CD8+ absolute counts have plateaued in the low-normal range. Lymphocyte function has remained normal. TCR Vbeta repertoire at 3.5 years of age demonstrated improvement but persistent skewed repertoire, with polyclonal Gaussian distribution in 17/28 probes, nine polyclonal probes with non-Gaussian distribution, and two showing oligoclonality. Most recent CD4+ T cell flow cytometry analysis performed at 4 years of age showed a low naive T cell compartment for age (CD45RA+RO-CD4+: 8%) with a predominant central and effector memory signature (CD45RA-RO+CD4+ 92%). Recent thymic emigrants were also found to be decreased with only 3.7% CD4+ T cells expressing recent thymic emigrant markers CD45RA+RO-CD4+CD31+. Clinically, the patient has thrived with normal liver function as assessed by transaminases (ALT, AST, GGT), albumin, and bilirubin. She has remained free of serious invasive infections. Immunoglobulin replacement therapy was required for 2.5 years following HCT, which was attributed to rituximab exposure during GvHD treatment, but was able to be discontinued at 3 years of age successfully. Finally, upon last assessment, the patient has demonstrated robust responses to diphtheria and tetanus vaccinations along with persistent 100% donor derived T cell chimerism. Ultimately, our patient is enrolled in school and continues to thrive living an age appropriate lifestyle.