Decision‐Making Regarding the Administration of Live Vaccines to Patients With a FOXN1 Heterozygous Missense Variant

FOXN1 is an essential transcription factor for thymic epithelial cell differentiation and consequently, naïve T cell development. Homozygous (Hom) FOXN1 variants cause T-lymphopenia, alopecia universalis, and nail dystrophy [1]. However, the phenotypic manifestations of heterozygous (Het) FOXN1 variants range from healthy to severe immunodeficiency. Herein we report the decision-making process regarding administration of live vaccines to an infant with a FOXN1-Het variant identified through newborn mass screening. A 21-day-old male neonate was suspected of having severe combined immunodeficiency (SCID) due to a low T-cell receptor excision circle (TREC) level (6.9 copies/μL [cut-off < 10 copies/μL]). There was no family history of immunodeficiency. He was born at 40 weeks of gestation with a body weight of 3282 g. He had no hair, facial, or nail abnormalities. A chest X-ray showed an indefinite thymic shadow. The blood test results were as follows: white blood cell count, 6.3 × 109/L (neutrophils, 38%; eosinophils, 4%; basophils, 0%; lymphocytes, 41%; and monocytes, 15%); IgG, 685 mg/dL; IgA, 9.4 mg/dL; IgM, 37.6 mg/dL; and IgE, < 2.0 IU/mL. Phytohemagglutinin-induced T-lymphocyte blastogenesis was normal (79,694 cpm; unstimulated controls, 649 cpm). Flow cytometric analysis of peripheral blood lymphocytes revealed the following: CD3+ T-cells (CD3+), 46% (normal range [NR], 53%–84%) and absolute count (AC), 1063/μL (NR, 2500–5500/μL); CD4+ T-cells (CD4+), 24% (NR, 35%–64%) and AC, 554/μL (NR, 1660–4000/μL); CD8+ T-cells (CD8+), 28% (NR, 12%–28%) and AC, 647/μL (NR, 560–1700/μL); CD31+CD45RA+ naïve CD4+ T-cells, 56% of CD4+ (NR, 61%–94%) and AC, 313/μL (NR, 1200–3600/μL); CD19+ B-cells, 25% (NR, 6%–32%) and AC, 578/μL (NR, 300–2000/μL); and CD56+CD16+ NK-cells, 5.36% (NR, 4%–18%) and AC, 1384/μL (NR, 170–1100/μL). Two subsequent rounds of TREC re-quantification revealed 0 copies/μL. Therefore, gene analysis for SCID was performed, revealing a Het-missense variant of FOXN1 (p.Arg320Trp; Figure 1A). The parent and the two siblings were healthy and did not undergo genetic testing. Based on the presence of naïve CD4+ T-cells, inactivated vaccines were administered but the rotavirus vaccine was withheld because of the low TREC results. After DPT-IPV vaccination, pertussis-specific antibody titers increased to protective levels, with FHA-IgG of 28 EU/mL and PT-IgG of 4 EU/mL (normal cutoff < 10 for both). The TREC level normalized after 8 months of age. Infections were treatable or resolved spontaneously (Figure 1B). The CD4+ counts remained >500/μL and the CD8+ counts exceeded normal lower limits. The naïve CD4+ counts increased transiently, followed by normalization of TREC levels (Figure 1B,C). At 16 months of age the child was administered live vaccines except for Bacillus Calmette–Guérin (BCG). Following the vaccination, antibody responses were detected by enzyme-immunoassay without adverse events: anti-measles IgG, 11.3; anti-rubella IgG, 2.1; and anti-varicella zoster IgG, 2.4 (all with a normal cutoff of < 2.0). The FOXN1 protein is composed of an N-terminal region, a Forkhead domain (FKD), which is important for DNA binding, and a C-terminal region [1, 2]. Hom-variants in the N-terminal region and the FKD cause hairless T-lymphopenia (Figure 1D), which will require a thymic implant or an alternative hematopoietic transplantation to reconstitute thymus tissue from donor thymic progenitors [2]. A patient with the Hom-variant of R320 located within the FKD underwent thymic transplantation due to severe immunodeficiency [3]. However, a patient with the p.R320W-Het variant was reported not to have severe infections [1]. Many FOXN1 Het-variants are currently classified as variants of uncertain significance. Het-variants with low TREC levels were located in positions affecting DNA-binding affinity and/or nuclear localization. Co-transfection experiments using wild-type and Het-variants suggested that FOXN1 protein functions as a monomeric transcription factor [2]. Missense heterozygous variants within the FKD may impair transcriptional activity, resulting in reduced TREC levels. In contrast, truncated FOXN1 proteins derived from frameshift variants downstream of the DNA-binding domain may act in a dominant-negative manner by antagonizing the DNA-binding capacity of the wild-type protein. Patients harboring such variants may develop SCID-like phenotype (Figure 1D) [4]. These genotype–phenotype correlations may also be useful in assessing prognosis. Although the number of T-cells increases over time in most patients with FOXN1-Het variants [2], administration of live vaccines should be determined based on the location of variants and individual immune function. Live vaccines possibly carry chronic viral infections and disseminated BCG in patients with combined immunodeficiency (CID). However, patients with a CD4+ count ≥ 500/μL or CD4+ percentages ≥ 15% of lymphocytes for at least 6 months are eligible for measles, mumps, rubella, and varicella vaccines in cases of partial DiGeorge syndrome or HIV [5]. A CD3+ count ≥ 500/μL, a CD8+ count ≥ 200/μL, and a normal mitogen response can also support relevance for MMR and varicella vaccination but not MMRV in CID [5]. In contrast, the BCG vaccine is unlikely beneficial and not recommended [5]. Therefore, the BCG vaccine was not administered to this patient. Two consecutive normalizations of the TREC level, a sustained CD4+ count > 500/μL after 1 year of age, and an increase in the IgG levels were helpful indicators for the administration of live vaccines. In conclusion, repetitive quantification of the TREC level not only served as a trigger for gene analysis but also helped decision-making regarding the administration of live vaccines to a patient with a FOXN1-Het variant. M.Y.-S. wrote the manuscript and Y.O. revised it and supervised the whole study process. Y.I. and T.O. treated the patient and collected the data. M.Y. organized newborn mass screening. All authors read and approved the final manuscript. I would like to thank Prof. Kohsuke Imai of the department of pediatrics, National defense medical college for his advice regarding vaccines. Informed consent for the publication of this report was obtained from the parents. The authors declare no conflicts of interest. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.