Platelet function abnormalities in KD were first reported by Yamada et al in 1978 (14). Studies in platelet activation markers in KD have subsequently been undertaken in Japan, USA, and Italy [Table.4]. Genetic risk factors and clinical presentation of KD differ among various populations and ethnicities. Hence platelet reactivity in KD can also be expected to be different in different populations. The important differences in clinical presentation of KD at Chandigarh, North India observed were the relatively higher rates of KD in children > 5 years of age compared to other population, and the relatively earlier occurrence of periungual peeling and thrombocytosis during acute phase of illness (15). Present study has been undertaken to analyse one of the most sensitive platelet activation markers, i.e. MPAs, by flow cytometry in a North Indian cohort of KD. To the best of our knowledge, MPAs has not been studied in acute and subacute phases of KD till date, and our study is perhaps the first attempt along these lines.
Levels of MPA% were found to be significantly elevated in children with KD compared to febrile controls and healthy controls in our study (median levels of 41.3%, 5.98%, and 4.48% respectively, p<0.001). MPA% levels significantly decreased after treatment with IVIg (median: 41.3% vs 18.5%, p <0.001) [Fig.2]. Elevations in MPA% suggest excess activation of platelets in KD compared to other common childhood febrile illnesses. Previous studies report that other platelet activation markers (e.g. neutrophil-platelet aggregates, platelet-derived microparticles, platelet VEGF levels, betathromboglobulin levels, PF4 levels, and platelet CD62P expression) are also elevated in acute phase of KD (7-13). Our study had a high proportion of children with incomplete KD (35.7%), which is in line with other recent studies from Asia (16, 17). Median duration of fever in our cohort of children with KD was 13.5 days that is higher compared to the European and North American cohorts (18, 19). Duration of fever may have impacted MPA% values in children with KD, as most of our patients were diagnosed more than 10 days after the disease onset when the platelet counts begin to increase. However, comparison of MPA% between children with fever duration ≤10 days and >10 days did not show any statistically significant difference. Ueno et al studied neutrophil-platelet aggregates (NPAs) in Japanese children with KD who had a median duration of fever of 4 days and had shown that NPAs% levels were significantly higher in KD compared to febrile controls (11).
Median levels of MPA% on follow-up at 3 months were lower compared to levels measured 24 hours after IVIg therapy, but higher than values obtained in age and sex-matched controls. It suggests that patients with KD may have a prolonged endotheliitis even after control of systemic inflammation with IVIg therapy. Laurito et al analyzed MPAs in patients with KD several years after the acute phase (mean interval- 76 months). Authors reported that CD41 expression at baseline and after ADP stimulation was significantly higher in patients compared to controls. However, MPA% levels were not significantly different between two populations (9). In a recent study by Yahata et al., platelet-derived microparticles were found to have rebound elevations in 8 of 14 patients with KD after discontinuation of aspirin (10). In our study also, we found rebound elevation in MPA% levels in 2 patients at 3 months of follow-up. However, majority of patients had lower levels even after discontinuation of aspirin.
Ueno et al. reported that levels of NPAs were significantly higher in patients with CAAs when compared with patients without CAAs (11). In our study, levels of MPA% were not different between patients with CAAs (n=3) and those without CAAs (n=11). Apparent discrepancy in results could be explained by differences in ethnicities of the study populations, small sample size in both studies, and differences in methodology of assessment of platelet activation. Ueno et al. also reported that levels of neutrophil-platelet aggregates were significantly lower in patients with KD treated with both IVIg and oral prednisolone than in patients who received IVIg alone (11). Corticosteroids were not used in any patients in our cohort.
A limitation of our study is the small sample size. This is understandable considering the fact that the protocol had to be completed in a limited span of time. Being a single centre study, there was uniformity in diagnosis and treatment protocols followed for management. Efforts were put in to standardize laboratory assays for MPAs before commencement of the study. We could use only single surface marker-CD14 for identification of monocytes in our study, as it is expressed both in classical and non-classical monocytes (20). CD14 can also be weakly expressed in some neutrophils and B cells, so, monocytes alone may not have been accurately captured in our experiment. Use of an imaging flow cytometry could have precisely showed the aggregates of monocytes and platelets (21), however, we could not use this modality in our study. Our patient cohort had a high proportion of children with KD who had fever of more than 10 days duration (64.3%). More studies are needed in future to decipher the platelet activation status in early stages of KD.
To conclude, MPA% was significantly elevated in our cohort of children with KD when compared with age and sex-matched febrile and healthy controls. Future long-term studies are warranted to find out whether elevated MPAs in KD would have any clinical implications. This may provide a theoretical basis for additional immunosuppressive and/or antiplatelet therapy in KD.