Zika virus (ZIKV) is a re-emerging virus that has recently spread into dengue virus (DENV) endemic regions and cross-reactive antibodies (Abs) could potentially affect ZIKV pathogenesis. Using DENV-immune serum, it has been shown in vitro that antibody-dependent enhancement (ADE) of ZIKV infection can occur. Here we study the effects of pre-existing DENV immunity on ZIKV infection in vivo. We infect two cohorts of rhesus macaques with ZIKV; one cohort has been exposed to DENV 2.8 years earlier and a second control cohort is naïve to flaviviral infection. Our results, while confirming ADE in vitro, suggest that pre-existing DENV immunity does not result in more severe ZIKV disease. Rather our results show a reduction in the number of days of ZIKV viremia compared to naïve macaques and that the previous exposure to DENV may result in modulation of the immune response without resulting in enhancement of ZIKV pathogenesis.

Zika virus (ZIKV) and dengue virus (DENV) are co-endemic in many parts of the world, but the impact of ZIKV infection on subsequent DENV infection is not well understood. Here we show in rhesus macaques that the time elapsed after ZIKV infection affects the immune response to DENV infection. We show that previous ZIKV exposure increases the magnitude of the antibody and T cell responses against DENV. The time interval between ZIKV and subsequent DENV infection further affects the immune response. A mid-convalescent period of 10 months after ZIKV infection results in higher and more durable antibody and T cell responses to DENV infection than a short period of 2 months. In contrast, previous ZIKV infection does not affect DENV viremia or pro-inflammatory status. Collectively, we find no evidence of a detrimental effect of ZIKV immunity in a subsequent DENV infection. This supports the implementation of ZIKV vaccines that could also boost immunity against future DENV epidemics.

Little is known about the contribution of virus-specific and cross-reacting antibodies (Abs) or the cellular immune response generated by a primary dengue (DENV) infection on the course of a secondary zika (ZIKV) infection in vivo. Here we show that the length of time between DENV/ZIKV infections has a qualitative impact on controlling early ZIKV replication. Depletion of DENV2-specific Abs in sera confirmed that those type-specific Abs do not contribute to ZIKV control. We show that the magnitude and durability of the neutralizing antibodies (nAbs) induced by a secondary ZIKV infection is modest compared to the response induced after a secondary heterologous DENV infection. Our in vivo results are showing a complex interplay between the cellular and innate immune responses characterized by a high frequency of plasmacytoid dendritic cells (pDC) correlating with an increase in the frequency of DENV antigen specific T cells and a significant control of ZIKV replication which is time dependent. Taken together, our results suggest that early after ZIKV infection other mechanisms such as the innate and cellular immune responses may play a predominant role in controlling ZIKV replication. Regardless of the time elapsed between infections there was no evidence of in vivo antibody-dependent enhancement (ADE) of ZIKV by DENV immunity. These findings have pivotal implications while interpreting ZIKV pathogenesis in flavivirus-experimented populations, diagnostic results interpretation and vaccine designs and schedules among others.

34The role of Zika virus (ZIKV) immunity on subsequent dengue virus (DENV) infections is 35 relevant to anticipate the dynamics of forthcoming DENV epidemics in areas with previous ZIKV 36 exposure. We study the effect of ZIKV infection with various strains on subsequent DENV 37 immune response after 10 and 2 months of ZIKV infection in rhesus macaques. Our results 38show that a subsequent DENV infection in animals with early-and middle-convalescent periods 39 to ZIKV do not promote an increase in DENV viremia nor pro-inflammatory status. Previous 40 ZIKV exposure increases the magnitude of the antibody and T cell responses against DENV, 41 and different time intervals between infections alter the magnitude and durability of such 42 responses-more after longer ZIKV pre-exposure. Collectively, we find no evidence of a 43 detrimental effect of ZIKV immunity in a subsequent DENV infection. This supports the 44 implementation of ZIKV vaccines that could also boost immunity against future DENV 45 epidemics. 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61Zika virus (ZIKV) is a re-emerging mosquito-borne Flavivirus that has captivated the 62 attention of the scientific community by its explosive spread in The Americas 1 , and severe 63 neurological sequelae following infection 2-4 . ZIKV established itself in tropical and sub-tropical 64 regions that are endemic to other closely-related flaviviruses such as dengue virus (DENV). 65Both viruses belong to the Flaviviridae family and are transmitted by Aedes spp. mosquitoes. 66DENV is a global public health threat, having two-thirds of world's population at risk of infection, 67 causing ~390 million infections annually 5,6 . DENV exists as four genetically similar but 68 antigenically different serotypes (DENV1-4) 7 . Exposure to one DENV serotype confers long-69 lived immunity against a homotypic secondary infection. However, secondary infection with a 70 heterologous serotype of DENV is the major risk factor to induce severe DENV disease [8][9][10] . 71Due to the antigenic similarities between DENV and ZIKV, concerns have been raised 72 regarding the impact of DENV-ZIKV cross-reactive immunity on the development of severe 73 clinical manifestations 11,12 . It has been demonstrated that DENV-immune sera from humans can 74 enhance ZIKV infection in vitro 13,14 , and in vivo in immune-deficient mouse models 15 . However, 75 recent results from our group and others have shown that previous flavivirus exposure-76including DENV-may have no detrimental impact on ZIKV infection in vivo in non-human 77 primates (NHP) 16,17 and humans 18 . Moreover, these studies and others suggest that previous 78 DENV immunity may play a protective role during ZIKV infection involving humoral and cellular 79 responses 19-23 . On the other hand, little is known about the opposite scenario, the role of a 80 previous ZIKV exposure on subsequent DENV infection, which is relevant to anticipate the 81 dynamics of forthcoming DENV epidemics. 82The recent ZIKV epidemic in the Americas resulted in t...