MyD88-dependent influx of monocytes and neutrophils impairs lymph node B cell responses to chikungunya virus infection via Irf5, Nos2 and Nox2

McCarthy, Mary K.; Reynoso, Glennys V.; Winkler, Emma S.; Mack, Matthias; Diamond, Michael S.; Hickman, Heather D.; Morrison, Thomas E.

In previous studies, we found that infection of mice with the attenuated and acutely cleared CHIKV strain 181/25 [14, 24] resulted in dLN hypertrophy, accumulation of lymphocytes, and formation of GCs [23] , all hallmarks of appropriate innate and adaptive immune responses to the infection. In contrast, the dLN of mice infected with pathogenic, persistent CHIKV strains became highly disorganized as early as 3 days post-infection (dpi), and failed to accumulate lymphocytes or develop GCs due in part to disruption of HEV-mediated lymphocyte recruitment [23] . Consistent with the robust response of the dLN to attenuated 181/25 infection, the presence of LNs is essential for clearance of 181/25 infection from joint-associated tissue (S1 Fig). To elucidate the mechanisms that lead to the rapid disorganization and function of the dLN following pathogenic CHIKV infection, we characterized cell populations at early times post-infection with pathogenic (parental strain AF15561) or attenuated (derivative strain 181/ 25) CHIKV, which differ by only five amino acids across the genome [25] . Pathogenic, but not attenuated CHIKV infection, resulted in increased numbers of monocytes (CD11b + Ly6C hi-Ly6G -) and neutrophils (CD11b hi Ly6C + Ly6G + ) in the blood and dLN within 24 h of infection (S2A- S2C Fig and Fig 1A-1C) . The influx of monocytes and neutrophils into the dLN preceded their arrival in the foot, the site of virus inoculation (S2D- S2F Fig) , which occurs between 3 and 5 dpi [26] . In contrast, few monocytes and neutrophils were present in the dLNs of mock-infected mice or mice inoculated with the attenuated CHIKV strain (Fig 1D) . Consistent with our flow cytometric analysis, immunofluorescence staining of dLN sections at 1 dpi detected a large population of Gr-1 + monocytes and neutrophils in the dLNs of mice infected with pathogenic CHIKV. These cells were mostly localized within the SCS and medullary sinuses, with a smaller population of monocytes and neutrophils within the B cell follicles (Fig 1D) . Thus, during pathogenic, but not attenuated, CHIKV infection, a rapid influx of monocytes and neutrophils into the dLN precedes its extensive disorganization.