Antibody-mediated enhancement aggravates chikungunya virus infection and disease severity

Lum, Fok-Moon; Couderc, Thérèse; Chia, Bing-Shao; Ong, Ruo-Yan; Her, Zhisheng; Chow, Angela; Leo, Yee-Sin; Kam, Yiu-Wing; Rénia, Laurent; Lecuit, Marc; Ng, Lisa F. P.

Whether antibodies are enhancing or neutralizing depends on numerous factors such as possible future infection with closely-related viruses (e.g., other alphaviruses), the virus strain, virus titer and concentrations, epitopes specificities, isotypes and FcγRs-binding affinities of the antibodies 22,58-60 . Patient plasma and mice serum samples used in this study at a low dilution factor have been previously shown to be strongly neutralizing 44, 46, 61, 62 and recognized epitopes located mainly on the CHIKV E2 glycoprotein 44, 46 . Furthermore, these CHIKV-specific antibodies were of IgG3 isotype 62 , and hence capable of moving across the placenta 63 . It remains to be seen if infants born to mothers infected with CHIKV during their pregnancy may suffer from a more severe disease due to the low levels of maternal-acquired CHIKV-specific antibodies. It was reported that newborn mice infected with DENV in the presence of maternally acquired anti-DENV antibodies had a more severe disease outcome 64 . Three-weeks old, C57BL/6 WT female mice (n = 5 animals per group) were infected via footpad inoculation with 10 6 PFU of CHIKV. Immediately, these mice were administered intraperitoneally with ~2 µg/ml (total IgG concentration) mice sera from CHIKV-infected mice (enhanced) or PBS (non-enhanced). Daily assessment of (a) viremia and (b) disease score were performed. All Data are presented as mean ± SD by Mann-Whitney U test (*P = 0.0278, *P = 0.0159, *P = 0.0159 and *P = 0.0297 for 2, 3, 4 and 5 dpi viremia respectively; **P = 0.0040, **P = 0.0079, **P = 0.0060, **P = 0.0079, **P = 0.0079, *P = 0.0104, *P = 0.0172, **P = 0.0040, **P = 0.0060, *P = 0.0106 and *P = 0.0106 for 3, 4, 5, 6, 7, 8, 9, 10, 11, 13 and 14 dpi disease score respectively). Immune-phenotyping was performed for (c) footpad and (d) popliteal lymph node on 6 dpi. Results are displayed as number of neutrophils, CD4+ T cells and monocytes per organ from 4 to 10 animals per group. (e) Total RNA was extracted from the joint footpads of infected mice (n = 5 to 10 animals per group) and qRT-PCR was performed to detect for the expression of crucial immune genes. Gene expression data are expressed as fold expression relative to the mock-infected mice. Mice sacrificed for immune-phenotyping and gene expression studies were infected as described for (a,b). All Data are presented as mean ± SD by Mann-Whitney U test (*P = 0.0186, *P = 0.0326 and *P = 0.0185 for neutrophils, monocytes and CD4+ T cells infiltration into respective organs; *P = 0.0276, *P = 0.0376 and *P = 0.0376 for IFNγ, DEF14 and IL-10 expression respectively). Using the joint footpad mouse model of CHIKV infection [41] [42] [43] [44] [45] [46] [47] [48] , disease severity was greatly enhanced in both WT and IFNαR+/− mice upon passive administration of low levels of CHIKV-specific antibodies. The exact mechanisms of this phenomenon remains to be fully elucidated. However as presented in this study, the disruption of host immune response could be a crucial driving force behind the augmented disease severity. This brings caution to the phenomenon that sub-neutralizing concentrations of virus-specific antibodies can enhance severity of infection and advocates the need for careful vaccine design and extensive pre-clinical trials. In fact, it was reported that CHIKV severity was increased in vaccinated mice that presented sub-optimal immune responses 65 . Unfortunately, it was also observed that a proportion of mice receiving