Table 2.

Cellular biomarkers proposed for the diagnosis, activity, and prognosis of sJIA and MAS.

Reference/Year	Cellular Biomarkers	Predictive Effects in sJIA and MAS
[40]. Grom, A.A.; Villanueva, J.; Lee, S.; Goldmuntz, E.A.; Passo, M.H.; Filipovich, A. Natural killer cell dysfunction in patients with systemic-onset juvenile rheumatoid arthritis and macrophage activation syndrome. J. Pediatr. 2003, 142, 292–296. https://doi.org/10.1067/mpd.2003.110	NK cells	Cytotoxic activity and NK cell counts were decreased, and a slight increase in perforin expression in CD8+ and CD56+ cytotoxic cells was found, in most patients with sJIA-MAS. NK cell dysfunction is a frequent immunological abnormality for sJIA-MAS and fHLH.
[41]. Villanueva, J.; Lee, S.; Giannini, E.H.; Graham, T.B.; Passo, M.H.; Filipovich, A.; Grom, A.A. Natural killer cell dysfunction is a distinguishing feature of systemic onset juvenile rheumatoid arthritis and macrophage activation syndrome. Arthritis Res. Ther. 2005, 7, R30–R37. https://doi.org/10.1186/ar1453		NK cytolytic activity was much lower in sJIA patients than in other JIA groups or controls. The subgroup of JIA patients who had not yet experienced any MAS event had low NK function and a lack of circulating CD56bright cells, similar to defects found in patients with MAS and HLH.
[42]. de Jager, W.; Vastert, S.J.; Beekman, J.M.; Wulffraat, N.M.; Kuis, W.; Coffer, P.J.; Prakken, B. Defective phosphorylation of interleukin-18 receptor beta causes impaired natural killer cell function in systemic-onset juvenile idiopathic arthritis. Arthritis Rheum. 2009, 60, 2782–2793. https://doi.org/10.1002/art.24750		NK cells from sJIA patients failed to phosphorylate the IL-18Rβ receptor following IL-18 stimulation, highlighting how NK cell dysfunction in sJIA is directly correlated with a defect in IL-18Rβ phosphorylation, which has important implications for understanding of the pathophysiological mechanisms and future therapy of sJIA.
[43]. Zhou, J.; Tang, X.; Ding, Y.; An, Y.; Zhao, X. Natural killer cell activity and frequency of killer cell immunoglobulin-like receptors in children with different forms of juvenile idiopathic arthritis. Pediatr. Allergy Immunol. 2013, 24, 691–696. https://doi.org/10.1111/pai.12130		In sJIA patients there was a decline in the activity of NK cells, which secreted more IFN-γ and less TNF-α, and the incidence of KIR2DS4 was reduced.
[44]. Put, K.; Vandenhaute, J.; Avau, A.; van Nieuwenhuijze, A.; Brisse, E.; Dierckx, T.; Rutgeerts, O.; Garcia-Perez, J.E.; Toelen, J.; Waer, M.; et al. Inflammatory Gene Expression Profile and Defective Interferon-γ and Granzyme K in Natural Killer Cells From Systemic Juvenile Idiopathic Arthritis Patients. Arthritis Rheumatol. 2017, 69, 213–224. https://doi.org/10.1002/art.39933		Increased IL-18 levels and a decreased IFN-γ/IL-18 ratio were found in sJIA patients. NK cells had an imbalance between inhibitory and activating receptors, with reduced G1 receptor expression and increased expression of NKp44. Hard-to-detect defects in the immune pathways governed by NK—such as the expression of granzyme K and the production of IFN-γ stimulated by IL-18—induced the immunoinflammatory dysfunctions that characterize sJIA.
[45]. Ohya, T.; Nishimura, K.; Murase, A.; Hattori, S.; Ohara, A.; Nozawa, T.; Hara, R.; Ito, S. Impaired Interleukin-18 Signaling in Natural Killer Cells From Patients With Systemic Juvenile Idiopathic Arthritis. ACR Open Rheumatol. 2022, 4, 503–510. https://doi.org/10.1002/acr2.11426		Increased levels of IL-18 induce the phosphorylation defects of MAPK and NF-κB in NK cells, and the inappropriate signaling of IL-18 in NK cells is directly related to the activity of sJIA.
[47]. Feng, D.; Huang, W.Y.; Niu, X.L.; Hao, S.; Zhang, L.N.; Hu, Y.J. Significance of Macrophage Subtypes in the Peripheral Blood of Children with Systemic Juvenile Idiopathic Arthritis. Rheumatol. Ther. 2021, 8, 1859–1870. https://doi.org/10.1007/s40744-021-00385-x	Macrophages	M1 induces inflammation in active sJIA, and M2a almost simultaneously triggers inflammation inhibition, while M2b and M2c play a major role in inhibiting inflammation in inactive sJIA. IL-6 had high levels in the group with active sJIA, while IL-4, IL-10, and IL-17 had high values in inactive disease.
[59]. Ter Haar, N.M.; Tak, T.; Mokry, M.; Scholman R.C.; Meerding, J.M.; de Jagerz, W.; Verwoerd, A.; Foell, D.; Vogl, T.; Roth, J. et al. Reversal of Sepsis-Like Features of Neutrophils by Interleukin-1 Blockade in Patients with Systemic-Onset Juvenile Idiopathic Arthritis. Arthritis Rheumatol. 2018, 70, 943–956. https://doi.org/10.1002/art.40442	Neutrophils	Neutrophils play an important role in sJIA, especially in the early inflammatory phase of the disease, and the neutrophil counts and the inflammatory activity in sJIA are both susceptible to IL-1 blockade.
[60]. Brown, R.A., Henderlight, M., Do, T., Yasin, S., Grom, A.A., DeLay, M., Thornton, S., Schulert, G.S. Neutrophils from Children with Systemic Juvenile Idiopathic Arthritis Exhibit Persistent Proinflammatory Activation Despite Long-Standing Clinically Inactive Disease. Front. Immunol. 2018, 9, 2995. https://doi.org/10.3389/fimmu.2018.02995		The results showed a higher percentage of the CD16+CD62Llo neutrophil population in active sJIA than in the control group. Serum concentrations of S100 alarm proteins (i.e., S100A8/A9 and S100A12) were strongly correlated with the number of neutrophils in the peripheral blood, and analysis of the entire neutrophil transcriptome identified 214 differentially expressed (SD) genes compared to the neutrophils from controls. Neutrophil activation in patients with active sJIA or CID, together with the pro-inflammatory gene expression blueprint, demonstrated prolonged activation of innate immunity.
[64]. Kim, J.-W.; Ahn, M.-H.; Jung, J.-Y.; Suh, C.-H.; Kim, H.-A. An Update on the Pathogenic Role of Neutrophils in Systemic Juvenile Idiopathic Arthritis and Adult-Onset Still’s Disease. Int. J. Mol. Sci. 2021, 22, 13038. https://doi.org/10.3390/ijms222313038		Neutrophils, including NETs, play a pivotal role in the pathogenesis of sJIA and AOSD as future clinical biomarkers for monitoring and prognosis.
[65]. Ravelli, A.; Minoia, F.; Davì, S.; Horne, A.; Bovis, F.; Pistorio, A.; Aricò, M.; Avcin, T.; Behrens, E.M.; De Benedetti, F.; et al. Expert consensus on dynamics of laboratory tests for diagnosis of macrophage activation syndrome complicating systemic juvenile idiopathic arthritis. RMD Open 2016, 2, e000161. https://doi.org/10.1136/rmdopen-2015-000161	Platelets	Platelet counts, followed by ferritin levels, AST, white blood cell counts, neutrophil counts, fibrinogen, and ESR, were selected for the early diagnosis of MAS in sJIA.
[67]. De Matteis, A.; Pires Marafon, D.; Caiello, I.; Pardeo, M.; Marucci, G.; Sacco, E.; Prencipe, G.; De Benedetti, F.; Bracaglia, C. Traditional Laboratory Parameters and New Biomarkers in Macrophage Activation Syndrome and Secondary Hemophagocytic Lymphohistiocytosis [abstract]. Arthritis Rheumatol. 2020, 72 (Suppl. S4). Available online: https://acrabstracts.org/abstract/traditional-laboratory-parameters-and-new-biomarkers-in-macrophage-activation-syndrome-and-secondary-hemophagocytic-lymphohistiocytosis/ (accessed on 21 August 2022).		Platelet count and ferritin are two relevant laboratory parameters, with high specificity and sensitivity, respectively, for the diagnosis of MAS in the context of sJIA.
[69]. Minoia, F.; Davì, S.; Horne, A.; Demirkaya, E.; Bovis, F.; Li, C.; Lehmberg, K.; Weitzman, S.; Insalaco, A.; Wouters, C.; et al. Clinical features, treatment, and outcome of macrophage activation syndrome complicating systemic juvenile idiopathic arthritis: a multinational, multicenter study of 362 patients. Arthritis Rheumatol. 2014, 66, 3160–3169. https://doi.org/10.1002/art.38802	Complete blood cell count	Typical laboratory features of sJIA include microcytic anemia; leukocytosis, thrombocytosis; elevated immunoglobulins; elevated ESR, CRP, and fibrinogen; and hypoalbuminemia.
[15]. Ravelli, A., Minoia, F.; Davì, S.; Horne, A.; Bovis, F.; Pistorio, A.; Aricò, M.; Avcin, T.; Behrens, E.M.; De Benedetti, F.; et al. 2016 Classification Criteria for Macrophage Activation Syndrome Complicating Systemic Juvenile Idiopathic Arthritis: A European League Against Rheumatism/American College of Rheumatology/Paediatric Rheumatology International Trials Organisation Collaborative Initiative. Arthritis Rheumatol. 2016, 68, 566–576. https://doi.org/10.1002/art.39332		MAS includes pancytopenia; increased levels of ferritin, liver enzymes (e.g., aspartate and alanine transaminases), triglycerides, and D-dimers; and hypofibrinogenemia.