Plasmodium knowlesi is an emerging zoonotic human malaria parasite of South East Asia which can cause severe and fatal malaria. The immunopathology of knowlesi malaria in humans is still largely unknown. Older individuals have demonstrated higher parasitaemias in previous clinical studies, which is associated with disease severity. Regulatory T cells (Treg) are key mediators of cellular immune responses and are known to change with age. We hypothesised that Treg activation would increase in knowlesi malaria, and would be greater in those with higher parasitaemia and in older individuals.
We used 10-colour flow cytometry to characterise the activation phenotype of CD25+Foxp3+CD127low CD4 Treg cells in uncomplicated knowlesi malaria adult patients presenting to two district hospitals in Sabah, Malaysian Borneo from 2010-2014. Acute and convalescent samples were analysed for 12 malaria patients. To investigate age association, samples were separated by patient age into two groups; young (15-25yrs) and older (>48yrs), with afebrile malaria-negative controls for each age group.
There was no statistically significant increase in Treg numbers in uncomplicated knowlesi malaria compared to healthy controls in either young or old individuals. However, compared to controls, a higher percentage of Tregs were activated (CD45RA-Foxp3hi) in acute malaria patients, and the proportion of activated Tregs was higher in older versus young patients (p=0.01). This age-related increase in Treg activation was also seen in malaria negative controls (p=0.03), despite no statistically significant difference in circulating Treg numbers. In older but not young patients, Treg activation was associated with expression of CXCR3, a marker of migration to peripheral sites of inflammation (p=0.03).
Preliminary results indicate that there is an age-associated change in Treg activation phenotype resulting in a more suppressive immune environment in older individuals, both for healthy controls and those with acute uncomplicated P. knowlesi malaria.