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 comparison with attenuated CHIKV infection, pathogenic CHIKV infection induces poor GC formation in the dLN and a diminished neutralizing serum Ab response [14, 23] . We hypothesized that the enhanced lymphocyte accumulation and organization in the dLN of mice depleted of monocytes and neutrophils prior to pathogenic infection would improve GC formation. To investigate this idea, mice were treated with a single dose of anti-Gr-1 or isotype control mAb one day prior to pathogenic CHIKV infection, and GC responses in the dLN were evaluated at 14 dpi by staining sections with the GC B cell marker GL7. Anti-Gr-1 mAbtreated mice had increased numbers of GCs per dLN, and the GCs were larger than those of the isotype control group (Fig 4A-4C) , resulting in an increased total GC area in the dLN ( Fig 4D) . Flow cytometry analysis also showed an increased percentage and number of CD95 + GL7 + GC B cells in the dLN of mice infected with pathogenic CHIKV and treated with anti-Gr-1 mAb (Fig 4E-4G) . Early depletion of monocytes and neutrophils also resulted in greater numbers of CD138 + IgD lo plasma cells (PCs) (Fig 4H-4J ) and increased the number of CHIKV-specific Ab-secreting cells (ASCs) in the dLN (Fig 4K) . The effects of anti-Gr-1 mAb treatment were most pronounced in the dLN, as we observed modest effects on the percentages of GC B cells and plasma cells in the left inguinal LN and the spleen (S4 Fig). Anti-Gr-1 mAb treatment had no effect on the amount of total CHIKV-specific IgG present in serum at day 7, 14, and 28 dpi (Fig 4L) . However, consistent with the effects of anti-Gr-1 mAb treatment on GC formation, we observed an increase in CHIKV neutralization activity in the serum of anti-Gr-1-treated mice compared with the isotype control group at 28 dpi (Fig 4M) . Together, these data demonstrate that early depletion of monocytes and neutrophils during pathogenic CHIKV infection improves the endogenous, polyclonal virus-specific GC B cell responses in the dLN and, despite predominantly impacting the dLN, leads to a modest but detectable improvement in serum Ab neutralization activity.