This study provided immunophenotypes related to the monocyte subtype by IVIG responsiveness for this disease. As innate immune cells initiate and propagate the immune responses, monocytes were therefore investigated. We revealed statistically lower proportions of intermediate monocytes in children with IVIG-resistant KD.
The monocyte population is heterogenous that plays an important role as the first line of immune defense and they migrate to inflammatory or infected tissues and can differentiate into macrophages or dendritic cells.[10, 11] Monocytes are recruited by chemokines (such as CD192 or CCDR2) that bind to receptors (CD14, CD16 or CD64) expressed on their cell surface. In human, we have 3 subtypes of monocytes which have different roles in immunity, the classical (CD14 + CD16-), the intermediate (CD14 + CD16+), and the nonclassical (CD14 + CD16++) monocytes.[12, 13] Each subtypes have characteristics unique to their key functions. The classical monocytes have high phagocytic ability, effectively produces inflammatory mediators in response to bacterial products, repairs tissue, and migrates to sites of inflammation. The intermediate monocytes are a newly defined subset which were previously identified within the nonclassical subset. They also produce inflammatory mediators in response to bacteria and has the ability to expand during infection and antigen presentation. The intermediate monocytes have proportionally increased in inflammatory and chronic conditions such as cardiovascular disease, rheumatoid arthritis, and Crohn’s disease, however, the role of intermediate monocytes in such disease has not been investigated.[14–18] Intermediate monocytes actively produce proinflammatory cytokines such as TNF-α, IL-1β and IL-6. Lastly, nonclassical monocytes produce TNF-α, IL-1β and CCL3 in response to viral and immune complex stimulation and they have the ability of FcƔ mediated phagocytosis.
Some researchers have demonstrated the role of monocyte/macrophage in KD. In affected tissues in autopsy cases and skin biopsy specimens of KD patients, infiltration of monocytes/macrophages is notable and the number of CD14 + monocytes/macrophages in peripheral blood augmented with increased activation of CD14 + CD23 + monocytes/macrophages during the acute stage of KD [20–22]. In addition, increased peripheral blood CD14 + monocytes/macrophages and secretion of TNF-α, IL-6, and IL-1 were observed more dominantly in KD patients with CAL [9, 22–25]. Koga et al. have demonstrated CD14 + monocytes/macrophages were activated in peripheral blood in the acute stage of KD with exaggerated phagocytosis and strong expression of TNF-α which became weakly expressed in the convalescent stage.
IVIG is a classic treatment modality in KD and those who received IVIG within 10 days of onset have decreased probability of having CAL, but if the patient is resistant to IVIG treatment, the patients have increased probability of developing CAL. The action mechanism of IVIG is not yet fully understood. IVIG plays a role in immune homeostasis by suppressing the activation of innate and adaptive immunity and inhibiting inflammatory mediators to enhance the anti-inflammatory processes [27, 28]. Das et al. have revealed that IVIG induces autophagy in peripheral blood monocytes, monocyte derived from dendritic cells, and M1 macrophages but not in M2 macrophages of healthy donors. Autophagy, a regulated mechanism for clearance of damaged/dysfunctional cells or components to warrant regeneration of new and healthy cells. This process plays a fundamental role in the regulation of innate and adaptive immune responses, lymphocyte differentiation, survival, and homeostasis [30–32]. Furthermore, the concept of autophagy is also important in regulating autoimmune and inflammatory diseases including systemic lupus erythematosus, inflammatory bowel diseases, rheumatoid arthritis, psoriasis, multiple sclerosis and myositis and some report autophagy as a potential target to treat for such diseases. [32–36]
According to current literatures, approximately 10–20% of patients are IVIG-resistant, and further research efforts are required to better characterize and predict IVIG-resistant patients [37–40]. Risk factors of IVIG resistance include younger age, male, higher neutrophil count, C-reactive protein (CRP), total bilirubin (TB), aspartate aminotransferase (AST), alanine aminotransferase (ALT) and lactate dehydrogenase (LDH). [2, 37–39, 41] Scores related to IVIG resistance have been developed using the above variables, but there are discrepancies of performance between Japanese cohorts and North American cohorts.
IVIG for acute KD decreases the absolute number of circulating monocytes/macrophages. NF-κB activation in peripheral CD14 + monocytes/macrophages also significantly decreased after IVIG therapy in acute stage of KD,  Katayama et al. also observed increased number of CD14 + CD16 + monocytes/macrophages in acute stage of KD which decreased after IVIG infusion.
In this present study, we have evaluated the monocyte subtypes to determine the role of it regarding IVIG responsiveness. The Intermediate monocytes in patients with KD were significantly higher than NFC but significantly lower than OFC. However, when we compared by IVIG responsiveness, the intermediate monocytes were significantly lower in the IVIG-resistant group compared to IVIG-responsive group. This implies that lower level of intermediate monocyte could play an important role in IVIG-resistant condition, as in interfering the action of IVIG.
After resolution of fever (after IVIG infusion), the intermediate monocyte decreased significantly in the IVIG-responsive group and while they were increased in the IVIG-resistant group. This finding may also imply some interaction between IVIG and the intermediate monocytes. While the changes of intermediate monocytes are more dramatic during the acute phase of KD, the level of intermediate monocytes became similar at convalescent phase of KD.
During ongoing pandemic, Coronavirus disease 2019 (COVID-19), multisystem inflammatory syndromes in children (MIS-C) has been confused to Kawasaki disease. Carter et al. have described peripheral immunophenotypes in MIS-C associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV2) infection. In the acute phase of MIS-C associated with SARS-CoV2, surface expression of CD14 was reduced while CD64 expression was increased, which implies activation and cytokine production. During the acute phase of COVID-19, the classical monocyte proportion was unchanged. The authors revealed that MIS-C appears distinct from KD because they could not observe increased monocyte counts.
This study had limitations. First, the small enrollment number of patients with IVIG-resistant, which could limit the statistical analyses. Second, the monocyte profiles between IVIG-responsive and -resistant in KD children were only assessed by the number/proportion of monocyte subtypes. Further functional studies, on the intermediate monocytes in particular, are needed. However, further evaluation into the intermediate monocytes could warrant a step forward to reveal the pathophysiology of KD, especially IVIG-resistant KD.
In conclusions, the intermediate monocyte may play an important role in IVIG responsiveness. Low starting levels of intermediate monocytes, followed by dramatic increase post-IVIG infusion during acute phase of KD are associated with IVIG-resistance. Functional studies on intermediate monocyte may help to reveal the pathophysiology of KD.