Mepolizumab treatment in a child with inherited TYK2 deficiency

To the Editor,

We report the case of a two-year-old child born to consanguineous parents of Algerian descent who, from the age of one month, developed recurrent severe respiratory viral infections (Fig. 1A). During the first two years of her life, she was hospitalized nine times in the intensive care unit for virus-induced acute respiratory distress, with pneumonia due to SARS-CoV-2, respiratory syncytial virus (RSV), coronavirus OC43, and influenza A virus. Each of these episodes was followed by hypereosinophilia (Fig. 1D). The patient soon developed recurrent episodes of severe acute respiratory failure with recurrent episodes of wheezing, without other clinical signs of atopy (Fig 1D). During each hospitalization for acute respiratory failure, she received systemic steroid treatment, leading to a decrease in eosinophil counts and respiratory improvement. From the age of four months onwards, the patient had hypereosinophilia (up to 10,000/mm³) but with normal total IgE levels (Fig 1D). Bilateral lung opacities were observed (Fig. 1C) and bronchoalveolar lavage at the age of eight months revealed no pathogens and the presence of 4% eosinophils (while under steroid treatment). Treatment with corticosteroids at a dose of at least 1 mg/kg/day was required to obtain an eosinophil count of 2,500/ mm³ after a 12-month period of recurrent hospitalizations, but the patient nevertheless continued to suffer from episodes of virus-induced respiratory distress. The patient had no atopic or parasitic disease that could explain the hypereosinophilia (toxocariasis serology and stool parasitology tests were negative), and hematological causes of hypereosinophilia were ruled out by an otherwise normal hemogram and a bone marrow aspirate that was normal (including karyotype and RT-MPLA transcripts: negative for JAK2, FIP1, BCR ABL) except for 14% hypereosinophilia. Blood immunophenotyping showed: 1.3% CD3⁺CD4⁺CD7⁻ (<physiological threshold of 3%), 0.4% CD3⁻CD4⁺CD7⁻CD2⁺CD5⁺ (<physiological threshold of 1%), and 1.5% CD3⁺CD4⁻CD8⁻TCR alpha and beta (<physiological threshold of 1.5%) cells. There was, therefore, no phenotypic evidence for a lymphoid origin of the eosinophilia. Immunophenotyping showed slightly low percentages of naïve CD4 and CD8 T cells and central memory CD8 T cells, a high percentage of memory effector CD8 T cells and a normal percentage of TEMRAs, mild B lymphocytosis with a high percentage of CD19⁺/CD27⁺ B cells, and NK lymphocytosis. Compared to aged-matched controls, deep immunophenotyping by CyTOF (Cytometry by Time of Flight) also showed a reduced counts of naïve CD4 (679/uL versus a mean of 1283/uL) and CD8 T cells (354/uL versus a mean of 607/uL), and an increased count of Th2 cells (CCR4+CCR6- CD4 T cells: 120/uL vs a mean of 43/uL). Functional assays showed normal T-cell proliferation in response to phytohemagglutinin (PHA) and OKT3 (VN>30%), but no T-cell proliferation in response to tetanus toxoid and candidin. IL-5 was undetectable in blood. Post-vaccination serological test results were normal for diphtheria, tetanus, pneumococcus, and Haemophilus influenzae. Complement levels were normal. A tryptase test was negative. IgG, IgA and IgM levels were within the normal ranges. IgE levels were 21 kIU/l (normal). We then performed panel sequencing for eosinophilia, which revealed a biallelic variant of TYK2 (encoding tyrosine kinase 2) that was also identified by whole-exome sequencing (WES), (c.3388C>T, p.Arg1130*). No other candidate variant was identified by WES. This variant was predicted to be loss-of function, due to the creation of a premature stop codon in the C-terminal part of TYK2, and it was not reported in GnoMAD V4.1.

Figure 1.

A: Family pedigree showing the segregation of the TYK2 mutant (M) allele. Double lines connect the two consanguineous parents. The closed black symbol indicates the proband (patient 1, P1) with TYK2 deficiency, and the open symbols indicate healthy family members. WT: wild-type.

Figure 1B: Western blotting of the EBV-B cells from P1, showing TYK2 deficiency. The TYK2, STAT1, STAT3, pSTAT1, pSTAT3 and GAPDH proteins are shown. The response to IFN-α was similar to that in patients with complete TYK2 deficiency.

Figure 1C: Computed tomography of P1 showing bilateral lung opacities.

Figure 1D: Initial outcome before mepolizumab treatment. RSV: respiratory syncytial virus; BAL: bronchoalveolar lavage; NIV: non-invasive ventilation; IV: intravenous. AOM: acute otitis media. ARDS: acute respiratory distress syndrome. NIV: non-invasive ventilation. VZV: varicella zoster virus.

Figure 1E: Outcome after mepolizumab treatment. RSV: respiratory syncytial virus; NIV: non-invasive ventilation; IV: intravenous; IV IgG: intravenous immunoglobulin; TMP-SMX: trimethoprim-sulfamethoxazole.

P1’s variant was confirmed by Sanger sequencing. Both her parents and her two sisters were heterozygous for the variant and her brother was WT and healthy. We investigated the functional impact of the variant, using EBV-B cells derived from P1. Residual amounts of TYK2 protein were detected on western blots, but at a slightly lower molecular weight. Nevertheless, responses to IL-23, IFN-α and IL-10 were as weak as those in a patient with complete TYK2 deficiency (Fig. 1B), suggesting that the patient displayed autosomal recessive (AR) complete TYK2 deficiency, with residual protein expression, as previously described[1]. TYK2 is one of the four human JAKs. It is involved in the IL-10, IL-12, IL-23 and type I IFNs (13 IFN-α subtypes, IFN-ω, IFN-β, IFN-ε, and IFN-κ) pathways. Complete TYK2 deficiency was first described in 2006 in a single patient and five forms of autosomal recessive (AR) TYK2 deficiency have now been described in 25 patients: (i) complete without and (ii) with residual expression, (iii) partial deficiency affecting all pathways, partial deficiency affecting specifically IL-23 signaling due to (iv) rare and (v) common variants. In these patients, impaired IL-12- and IL-23-mediated IFN-γ production underlie mycobacterial diseases due to tuberculous and non-tuberculous mycobacteria. Like patients with IL-12Rβ1 deficiency, in whom IL-12- and IL-23-mediated IFN-γ production is abolished, some TYK2-deficient patients are also susceptible to intramacrophagic pathogens (Salmonella). Their IL-23-dependent induction of IL-17 is also weak, accounting for their fungal diseases (Candida). Impaired responses to type I IFNs underlie severe viral diseases, including COVID-19 pneumonia, influenza pneumonia, herpes simplex encephalitis and adverse reactions to live attenuated vaccines. Impaired responses to IL-10 seem to be clinically silent. Incomplete clinical penetrance has been observed for mycobacterial and viral diseases, as 48% and 60% of patients, respectively, develop these diseases. Deep immunophenotyping revealed no PBMC abnormalities in patients with the various forms of TYK2 deficiency, indicating the presence of normal numbers and percentages of the different myeloid and lymphoid cell subsets (purely adaptive T cells (CD4⁺ T, CD8⁺ T cells and their subsets), innate-like adaptive T cells (γδ T, mucosal-associated invariant T, and invariant natural killer [NK] T cells), innate lymphoid cells (NK, innate lymphoid cell progenitors [ILCP], and ILC2), monocytes, and dendritic cells, in the three TYK2-deficient patients and in our patient.

Eosinophil levels were rarely mentioned[2]. Our patient presented virus-triggered lung hyperreactivity and severe hypereosinophilia with very deleterious effects on her quality of life. She did not suffer from the mycobacterial, fungal or bacterial infections described in previously reported patients[3], but her susceptibility to a broad range of viral diseases was explained by defective type I IFN responsiveness. This defect was probably a triggering factor in her secondary lung hyperreactivity, which became her main condition, perhaps driven by defective Th1 and excessive Th2 differentiation, as previously suggested[3]. Given the patient’s history of severe viral diseases, preventive management with infusions of polyvalent immunoglobulin (IVIg) and trimethoprim-sulfamethoxazole (TMP-SMX) in addition to inhaled corticosteroids was initiated (Fig. 1E). IL-5 and IL-6 plasma levels were normal. Other cytokine levels were not measured. However, due to the dependence on systemic corticosteroids and the recurrent pulmonary symptoms and hypereosinophilia, we decided to initiate targeted corticosteroid-sparing therapy to block interleukin 5 (IL-5) (Fig. 1E)[4]. Mepolizumab, used for the treatment of severe eosinophilic asthma, eosinophilic granulomatosis, and hypereosinophilic syndrome, was started at the age of 20 months, at a dose of 40 mg per month delivered subcutaneously. This treatment was well tolerated clinically. Two weeks after the first injection, eosinophil counts had fallen strongly, to 300/mm³, reaching normal levels one month later. The patient suffered from chickenpox due to VZV infection, leading to a suspension of oral steroid treatment, and had one episode of respiratory distress due to infection with Parainfluenza virus and Mycoplasma pneumoniae but without hypereosinophilia after treatment initiation. This patient with complete TYK2 deficiency, who suffered from severe viral infections and severe hypereosinophilia causing wheezing respiratory disease, is now three years old and remains clinically well on mepolizumab and IVIg treatment, with an eosinophil count of 140/mm³ (Fig. 1E).

Thus, we report the case of a patient with complete TYK2 deficiency and virally induced hypereosinophilia and respiratory failure who responded to IL-5 blockade. Routine assessment of circulating levels of IL-5 and/or deep-immunophenotyping showing a clear skewing towards a Th2 phenotype can guide therapeutic intervention towards the use of targeted therapies, such as treatments targeting IL-5. The mechanism underlying this abnormal skew towards a Th2-phenotype has yet to be fully elucidated, but studies in mice suggest that Tyk2 may be involved in regulating the Th1/Th2 balance in favor of Th1 and downregulating eosinophil recruitment in the airway[5]. A similar mechanism may be at work in our patient.

Clinical Implications:

We report severe hypereosinophilia and lung disease in a patient with inherited complete TYK2 deficiency who benefited from mepolizumab therapy.

Abbreviations:

RSV

Respiratory syncytial virus

SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2

JAK2

Janus kinase 2

FIP1L1

Factor interacting with PAPOLA and CPSF1

CyTOF

Cytometry by Time of Flight

TEMRA

Terminally differentiated effector memory

PHA

Phytohemagglutinin

IFN

Interferon

IVIg

Polyvalent immunoglobulin

TMP-SMX

Trimethoprim-sulfamethoxazole