Platelets Can Phagocytose Influenza Virus Which May Contribute to the Occurrence of Thrombocytopenia during Influenza Infection

Thrombocytopenia is a common and sometimes life-threatening symptom in patients with influenza, which indicates that platelets may play a significant role during virus infection. However, the mechanisms underlying influenza associated thrombocytopenia are still unclear to date. In this study, the relationship between platelets and influenza infection was studied in vivo and in vitro.

In the period 2009-2012 we randomly measured viral load and platelet count in laboratory-confirmed influenza patients (A/H1N1 subtype; n=85) presented at the emergency department of Erasmus MC. In a cross-sectional study we found, after excluding patients with HIV infection, chemotherapy treatment and immediate referral to the ICU, that a high viral load was significantly correlated with a low platelet count at presentation (n=36; Spearman correlation coefficient 0.341; P=0.042). To study this relationship further we inoculated ferrets (n=120) with an H3N2 (n=30), pandemic H1N1 (n=30), or H5N1 influenza A virus (n=30) and measured platelet counts in six ferrets per time point at day 0, 1, 2, 3 and 4 after infection. Thrombocytopenia was seen at day 3 in all ferrets compared to the control animals (n=30). The decrease in platelet counts was significant in the H5N1 infected ferrets (p<0.05). For the sialidase and influenza virus inhibitor oseltamivir, we have previously shown that it functions as an inhibitor of platelet degradation. Interestingly, thrombocytopenia in ferrets could also be inhibited by oral administration of oseltamivir. However, these results were not statistically significant.

Finally, we studied the interaction between platelets and influenza viruses by electron microscopy. In a platelet suspension incubated for 1 minute with influenza virus we observed platelets with virus containing vacuoles, suggesting that the platelets had rapidly phagocytosed the viruses. Characteristics of these platelets incubated with influenza virus were measured including membrane expression of the GPIb-IX-V complex, CD62P, and surface glycans sialic acid and GlcNac using flowcytometry and functional capacity employing aggregometry using collagen, thrombin and ADP as agonists. However, no effect was seen in expression of GPIba and GPIX, P-selectin expression and surface expression of sialic acid and GlcNAc after incubation with influenza virus. Also no effect was seen on functional capacity of these platelets.

Our study shows that influenza virus is significantly correlated with a lower platelet count. Phagocytosis of influenza virus by platelets may play an important role in the occurrence of thrombocytopenia during influenza infection and may be a mechanism of virus clearance during infection.