Platelet factor 4 enables platelets to suppress early parasite growth in a mouse model of blood stage malarial infection

Platelets are best known for their roles in hemostasis, but there is accumulating evidence for their importance in immunity and host defence against infection. We have shown that platelets are required for host protection during malarial infection and are a major determinant for survival. Platelets control the growth of intraerythrocytic Plasmodium parasites by directly binding to infected erythrocytes and killing the parasite inside. We and others have also described the parasite killing mechanism exerted by platelets. Upon binding to parasite-infected red cells, platelets release a protein with dual chemokine and antimicrobial functions called platelet factor 4 (PF4). PF4 binds to the red cell Duffy antigen chemokine receptor, and is then absorbed into the parasite where it lyses the parasite food vacuole and kills the parasite. This work aimed to further investigate the role of PF4 in vivo using mice a PF4 knockout mouse strain (Pf4tm1Mako/tm1Mako) and blood stage infection with a murine malaria model parasite, P. chabaudi. We confirmed that the homozygous mutant mice did not produce any PF4 protein. Following challenge with P. chabaudi (1 x 10⁴ parasites administered intravenously), rates of survival amongst cohorts of age and sex matched wild type and homozygous mice (both on a C57BL/6 background) were not significantly different (approximately 30% for females and 10% for males). However, a deficiency of PF4 resulted in significant differences in parasite growth during the pre-peak parasitemia phase of the infection. Between days 5 and 8 following infection, proportions of parasite-infected red cells in the circulation of PF4-null mice were double those in wild-type mice. This coincided with a 50-60% reduction in the frequency of growth arrested or dead parasites (detected using terminal deoxynucleotidyl transferase dUTP nick end labelling, also known as TUNEL labelling). Together these data indicate that platelet-derived PF4 has an important in vivo malaria-protective role. The protein is necessary for the platelet to control parasite growth during the early stage of erythrocytic malarial infection. However, the highly virulent nature of the P. chabaudi strain used in these experiments precluded detection of any predicted survival disadvantage in the PF4-null mice.