Plasmodium knowlesi is an emerging zoonosis in Southeast Asia which can cause severe and fatal disease, particularly in older adults. Parasite biomass in human disease increases with increasing age, and both age and parasitaemia are independent risk factors for disease severity. The reasons for the effects of aging are not clear. Regulatory T cells (Treg) are key mediators of cellular immune responses that change in frequency and activation with age. We hypothesised that Treg activation would increase in knowlesi malaria, and would be greater in older individuals.
We characterised the activation phenotype of CD25+Foxp3+CD127low CD4 Treg in young (15-25yrs, n=14) and older (>48yrs, n=11) adults with uncomplicated knowlesi malaria and comparable parasitemia, in Sabah, Malaysian Borneo; 20 age-matched afebrile, Plasmodium-negative controls; and 5 older severe knowlesi malaria patients.
Older individuals (controls and malaria patients) had less naïve-Treg. In acute malaria, Treg transiently expressed markers of activation, including surface expression of CTLA-4 and TNFR2. In particular, there were more TNFR2+ Treg in older uncomplicated patients than in young (p=0.02), and even more in severe malaria. Twenty-eight days following anti-malarial treatment, TNFR2 expression was still higher in older compared to younger patients (p=0.01). Expression of T cell inhibitory CTLA-4 was significantly higher in patients with severe malaria compared to those with uncomplicated malaria of the same age-group (p=0.04). Interestingly, compared to controls, heightened expression of the migration marker CXCR3, was seen only in older patients (p=0.03). There was a significant decline in Treg count in severe, and not uncomplicated disease compared to healthy controls (p=0.03).
Treg are activated in knowlesi malaria and most activated in severe disease. Aging-associated changes in the Treg activation phenotype may promote enhanced immune suppression in older individuals which could contribute to higher parasitaemia and a higher risk of severe knowlesi malaria in this age group.