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Background SCID is a syndrome characterized by profound T cell deficiency. BCG vaccine is contraindicated in SCID patients. Because most countries encourage BCG vaccination at birth, a high percent of SCID patients are vaccinated before their immune defect is detected. Objectives To describe the complications and risks associated with BCG vaccination in SCID patients. Methods An extensive standardized questionnaire evaluating complications, therapeutics, and outcome regarding BCG in patients diagnosed with SCID was widely distributed. Summary statistics and association analysis was performed. Results Data on 349 BCG vaccinated SCID patients from 28 centers in 17 countries was analyzed. Fifty-one percent of the patients developed BCG complications, 34% disseminated and 17% localized (a 33,000 and 400 fold increase, respectively, over the general population). Patients receiving early vaccination (≤ 1 month) showed an increased prevalence of complications (p=0.006) and death due to BCG complications (p<0.0001). The odds of experiencing complications among patients with T cells ≤ 250/uL at diagnosis was 2.1 times higher (95% CI, 1.4-3.4; p = 0.001) than among those with T cells > 250/uL. BCG complications were reported in 2/78 patients who received anti-mycobacterial therapy while asymptomatic and no deaths due to BCG complications occurred in this group. In contrast 46 BCG-associated deaths were reported among 160 patients treated with anti-mycobacterial therapy for a symptomatic BCG infection (p<0.0001). Conclusions BCG vaccine has a very high rate of complications in SCID patients, which increase morbidity and mortality rates. Until safer and more efficient anti-tuberculosis vaccines become available, delay in BCG vaccination should be considered to protect highly vulnerable populations from preventable complications.

Summary Tuberculosis (TB), caused by Mycobacterium tuberculosis (M.tb) and a few related mycobacteria, is a devastating disease, killing more than a million individuals per year worldwide. However, its pathogenesis remains largely elusive, as only a small proportion of infected individuals develop clinical disease either during primary infection or during reactivation from latency or secondary infection. Subacute, hematogenous, and extrapulmonary disease tends to be more frequent in infants, children, and teenagers than in adults. Life-threatening primary TB of childhood can result from known acquired or inherited immunodeficiencies, although the vast majority of cases remain unexplained. We review here the conditions conferring a predisposition to childhood clinical diseases caused by mycobacteria, including not only M.tb but also weakly virulent mycobacteria, such as BCG vaccines and environmental mycobacteria. Infections with weakly virulent mycobacteria are much rarer than TB, but the inherited and acquired immunodeficiencies underlying these infections are much better known. Their study has also provided genetic and immunological insights into childhood TB, as illustrated by the discovery of single-gene inborn errors of IFN-γ immunity underlying severe cases of TB. Novel findings are expected from ongoing and future human genetic studies of childhood TB in countries that combine a high proportion of consanguineous marriages, a high incidence of TB, and an excellent clinical care, such as Iran, Morocco, and Turkey.

Objective: To determine the proportion of children with herpes simplex encephalitis (HSE) displaying TLR3 deficiency, the extent of TLR3 allelic heterogeneity, and the specific clinical features of TLR3 deficiency. Methods:We determined the sequence of all exons of TLR3 in 110 of the 120 patients with HSE enrolled in our study who do not carry any of the previously described HSE-predisposing mutations of TLR3 pathway genes (TLR3, UNC93B1, TRIF, TRAF3, and TBK1). All the new mutant TLR3 alleles detected were characterized experimentally in-depth to establish the causal relationship between the genotype and phenotype.Results: In addition to the 3 previously reported TLR3-deficient patients from the same cohort, 6 other children or young adults with HSE carry 1 of 5 unique or extremely rare (minor allele frequency ,0.001) missense TLR3 alleles. Two alleles (M374T, D592N) heterozygous in 3 patients are not deleterious in vitro. The other 3 are deleterious via different mechanisms: G743D1R811I and L360P heterozygous in 2 patients are loss-of-function due to low levels of expression and lack of cleavage, respectively, and R867Q homozygous in 1 patient is hypomorphic. The 3 patients' fibroblasts display impaired TLR3 responses and enhanced herpes simplex virus 1 susceptibility. Overall, TLR3 deficiency is therefore found in 6 (5%) of the 120 patients studied. There is high allelic heterogeneity, with 3 forms of autosomal dominant partial defect by negative dominance or haploinsufficiency, and 2 forms of autosomal recessive defect with complete or partial deficiency. Finally, 4 (66%) of the 6 TLR3-deficient patients had at least 1 late relapse of HSE, whereas relapse occurred in only 12 (10%) of the total cohort of 120 patients.Conclusions: Childhood-onset HSE is due to TLR3 deficiency in a traceable fraction of patients, in particular the ones with HSE recurrence. Mutations in TLR3 and TLR3 pathway genes should be searched and experimentally studied in children with HSE, and patients with proven TLR3 deficiency should be followed carefully. TLR3 is one of the most highly conserved TLRs in humans that have evolved under the strongest purifying selection.1 TLR3 recognizes double-stranded RNA (dsRNA), a by-product produced during the viral replication of most viruses, including herpes simplex virus 1 (HSV-1).2 The most common known clinical consequence of human TLR3 deficiency is childhood herpes simplex encephalitis (HSE). Childhood HSE is a rare life-threatening complication of primary infection with HSV-1, a common neurotropic dsDNA virus that is innocuous in most children.3 HSE is the most common form of sporadic viral encephalitis in Western countries. 4,5 The pathogenesis of HSE had long remained unclear. Our recent studies have demonstrated that HSE may result from singlegene inborn errors of TLR3-mediated immunity in some children, 6 with homozygous or

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