2.2. Current and emerging biomarkers in diagnosis and treatment
2.2.1. Clinically-used biomarkers
Allergy diagnosis relies on evaluating the history of allergen exposure, symptomatology, and tests to determine sensitization. Elevated total serum IgE (tIgE) is associated with allergic disease, parasitosis and specific immunologic abnormalities. It has traditionally been used as a marker for atopy/allergy, prompting further evaluation.
Allergen-specific IgE (sIgE) is detected on MCs (skin prick testing) and in serum. sIgE can be detected against whole extracts as well as components obtained recombinantly or purified/native. Identifying cross-reactivity and primary sensitization aids treatment decisions such as allergen-specific immunotherapy (AIT).17 The glycosylation patterns of sIgE (and sIgG) have biomarker potential regarding allergy diagnosis, severity and treatment efficacy (tolerance induction). Sequential epitope-specific IgE-profiling appears valuable in peanut oral immunotherapy (OIT)18. Lower sIgE to food allergens at baseline was related to increased efficacy of peanut OIT.19 Allergen provocation testing is the gold standard when clinical history is inconclusive.20
Indirect sIgE-detection can be performed with a flow-cytometry-based functional assay that measures basophil activation after allergen exposure (basophil activation test, BAT). This test discriminates between sensitization and allergy for peanut and red meat allergies.21-23 The value of BAT extends to diagnosing various conditions, such as drug hypersensitivity, encompassing cases like IgE-based anti-tumour therapy in cancer patients24, as well as allergies to hymenoptera venom.21 BAT has been optimized for broader use.21 Passive MC activation testing (pMAT) can provide additional information, for example, when basophils are non-responsive to allergens. Ratio analysis of sIgE to tIgE can aid in clinical interpretation.25
Serum tryptase is a biomarker for anaphylaxis. Tryptase is released by activated MCs in 30 min to 2h. Elevated tryptase in the acute phase confirms the diagnosis, but normal levels do not exclude anaphylaxis.26,27 Basal serum tryptase is also increased in certain MC disorders.
Eosinophilia, or elevated absolute eosinophil count (AEC), is found in atopy, T2-high asthma, atopic dermatitis (AD), eosinophilic gastrointestinal disorders and delayed drug hypersensitivity reactions (DHR), although it is not specific. Eosinophilia is also observed in parasitic infections and autoimmune diseases. T2-high asthma includes, among others, allergic and eosinophilic asthma pheno-endotypes. Allergic asthma is characterized by increased circulating allergen-specific IgE, while eosinophilic asthma can be diagnosed based on increased blood (>300/uL) or sputum eosinophil numbers (³3%). On the other hand, T2-low asthma, including neutrophilic or paucigranulocytic asthma is characterized by low IgE, AEC, and allergic symptoms.28 These phenotypes have different severity and treatment implications in adults and children.15,29 AEC predicts therapeutic response to biologics in severe asthma (e.g., anti‐IL‐5).30,31
Fractional excretion of nitric oxide (FeNO) is a reproducible and non-invasive indirect biomarker of IL-13-mediated T2-airway inflammation. Higher values of FeNO are found in T2-high compared with T2-low asthma and help confirm diagnosis in adults and children. It predicts response to inhaled corticosteroids (ICS).32 Elevated FeNO values during ICS therapy do not support ICS dose reduction. Low FeNO values alone do not exclude bronchial asthma. FeNO levels vary with different asthma biologics.33
2.2.2. Emerging cellular markers
Allergen-specific Th2A-cells, found in allergic individuals, are CD4+CRTH2+CD161highHPGDS+CD27lowCD49dhighST2high memory cells that play a pathogenic role in AD, food allergy, asthma, and eosinophilic esophagitis (EoE).34 Th2A-cell frequency inversely correlates with treatment efficacy. CD38+Th2A-cells are an emerging clinical biomarker in T2-high asthma, with CD38 upregulation in Th2A-cells and downregulation with immunotherapy treatment.35
MCs are activated in allergic diseases like asthma and AD, with MAS-related G protein-coupled receptor-X2 (MRGPRX2) being a new biomarker for allergic disorders such as asthma and drug allergy. Serum MRGPRX2 levels are elevated in allergic asthma, especially in those responding well to ICS.36 This receptor is expressed on cutaneous MCs in patients with severe chronic spontaneous urticaria.37 Some reactions to specific drugs such as fluoroquinolone antibiotics or neuromuscular blocking agents have been postulated to be induced by MC MRGPRX2-mediated mechanisms.38 MC activation tests (MAT) are being developed to aid diagnosis and monitoring of allergic diseases.39 However, MC activation is not always present, requiring differential diagnosis.
T2-innate lymphoid cells (ILC2) contribute to inflammation in allergic disorders, such as allergic rhinitis (AR), chronic sinusitis, asthma and AD, by enhancing the activity of Th2-cells, eosinophils and their cytokines. ILC2 are increased in the blood, lung and skin of individuals with these conditions and are related to disease severity and treatment response.40 Notably, allergen-specific immunotherapy reduced frequencies of sputum ILC2s in patients with grass-pollen allergic rhinitis and asthma.41
2.2.3 Emerging type-2 cytokines and proteins
Allergic patients generally present higher serum T2-cytokine levels compared to healthy individuals. However, they are not routinely used as clinical biomarkers in allergy due to usually low serum detectable levels.
42 Elevated IL-4 differentiates
T2-high from T2-low asthma, while elevated IL-4 and IL-5 distinguishes asthma persistence in adults and children, respectively. IL-13 and IL-33 correlate with asthma severity, while thymic stromal lymphopoietin (TSLP) levels are increased in the skin of AD patients and in the airways of patients with severe asthma. Decreased T2-cytokines are associated with treatment success.
42,43Elevated blood levels of CC-chemokine ligand-22 (CCL22) have been detected in AD patients compared to controls and are associated with disease severity.42 Eotaxins (CCL11) are proposed biomarkers for AR, asthma and AD.42 Periostin is a biomarker of T2-inflammation in adults; however, its levels vary in children until bone growth stops.44 Eicosanoids play a role in allergy pathomechanisms. Levels of leukotriene B4 (LTB4) were increased in patients with asthma.32 Increased lipocalin-2 after sublingual immunotherapy was associated with clinical benefit.45 Additionally, prostaglandin E2 (PGE2) was upregulated in untreated allergic rhinitis and asthma patients with a significant decrease observed after AIT. Notably, PGE2 levels correlated with T2-inflammation and clinical markers, such as IL-13, sputum eosinophil counts and symptom scores.41 Lastly, decorin, an extracellular matrix proteoglycan participates in the pathogenesis of allergic asthma by reducing bioavailability of transforming growth factor beta (TGF-b).46
2.2.4 Emerging genetic and molecular omics markers
Genomic loci, such as 17q21 are associated with allergic diseases. This region is associated with gene expression of ORMDL sphingolipid biosynthesis regulator 3 (ORMDL3), an inhibitor of de novo sphingolipid synthesis, a mediator in severe allergic asthma.47 ORMDL3 is expressed in airway smooth muscle cells, airway epithelium, CD4 T-cells, and eosinophils. ORMDL3 expression is related to exaggerated T2-inflammation, increased expression of adhesion molecule ICAM-1, enhanced glycolysis and pro-inflammatory cytokine production (IL-6 and IL-18) in both structural lung cells and infiltrating immune cells in the lungs.48 Filaggrin (FLG) genetic variations are genetic biomarkers in eosinophilic asthma49 and AD,50 related to a higher risk of presenting Th2 multimorbidity.
Polygenic risk scores combining 41 genetic polymorphisms show significant associations with asthma risk.51 HLA-DR1 is abundant in cat allergy, while HLA-DR4 is associated with Alt a 1 responsiveness in Alternaria allergy.52 Notably, single nucleotide polymorphism (SNP) on D2HGDH (rs34290285) demonstrated significance in both asthma and allergic diseases.53 HLA-B alleles are related to drug hypersensitivity.54,55 Forty-two genetic loci are associated with AR.55 MicroRNAs (miRNAs), are small non-coding RNA molecules involved in gene expression regulation. Several miRNAs play a role in allergic diseases and have been proposed as potential biomarkers of both disease pathology and therapy outcomes.56 miR-155 plays a role in AR, AD, and asthma.56 miR-3935, the predicted target of PGE2 receptor (PTGER3), was upregulated during AIT in patients with allergy.41 The detailed role of miRNAs in allergy and asthma is reviewed elsewhere.56
Iron metabolism is involved in childhood allergic asthma. Downregulated solute carrier family-40 member-1 (SLC40A1) expression correlates with T2-inflammation in the lung and is used to classify patients into T2-low and T2-high subgroups. Decreased SLC40A1 results in reduced iron levels in the airways.57
Omics technologies can further explore allergy pathobiology, define specific endotypes, and refine disease classification and treatment.12
2.3. Therapeutic approaches
Antihistamines, decongestants and corticosteroids relieve allergy symptoms. AIT shifts the Th2-response to a Th1-response, and induces regulatory T- (Tregs) and B-cells (IL-10+ B-cells), subsequently promoting tolerance to allergens and providing long-term symptom relief.58 AIT is effective across all age groups, requiring a minimum of 3 years of treatment.31 Randomized controlled trials (RCTs) demonstrated AIT’s efficacy in children (≥5 years) and older patients (>65 years).59 Individual molecular sensitization profiles influence AIT effectiveness.60 AIT is the only treatment with maintained efficacy after stopping treatment. AIT can be combined with biological therapies (treatment antibodies) for “difficult-to-treat” allergic asthma or to increase tolerability.61 Biological therapies targeting IgE, interleukins, IL-receptors or TSLP (Table 2) are used to treat various allergic disorders.30,31 Desensitization protocols effectively prevent DHR, maintaining first-line treatment when no equivalent alternatives exist in drug allergy patients.62