INTRODUCTION
In 2015–2016, Zika virus (ZIKV) emerged in dengue virus (DENV) endemic regions of the Americas, causing 1.6 million infections and at least 11,000 cases of newborn microcephaly in Brazil (
1,
2). ZIKV is transmitted by mosquitoes and congenitally. Infections in pregnancy lead to the greatest disease burden of neurodevelopmental delays and microcephaly in 5%–8% of Zika-exposed newborns (
3). A primary concern in the development of vaccines and immunotherapies is that ZIKV co-circulates with other flaviviruses such that it is possible for most of the at-risk population to be sequentially exposed to diverse flaviviruses (
4,
5).
Flaviviruses have highly conserved and immunodominant envelope (E) proteins, such that an infection can generate cross-reactive antibodies against another related virus (
6–9). While some cross-reactive antibodies have no effect or even protect from subsequent disease, others can mediate antibody-dependent enhancement (ADE) of subsequent viral infection and worsen disease (
10–14). ADE can occur when immune complexes of flavivirus virions and cross-reactive IgG interact with Fcγ receptors on susceptible cells, facilitating virus uptake into host cells (
14–19). For example, pre-existing ZIKV immunity worsens subsequent DENV-2 and DENV-3 disease (
20). Also, DENV infections are predicted to provide temporary cross-protection against ZIKV disease followed by a period of enhanced risk (
21). Thus, it is important to understand how cross-reactive antibodies arise over multiple flavivirus infections.
Typically, primary infection generates flavivirus-reactive antibodies and long-lived memory B cells (MBCs) (
22–25). Upon subsequent infection, pre-existing MBCs and newly stimulated B cells can rapidly differentiate into antibody-secreting cells which are called plasmablasts (PBs) (
23,
25–27). Flavivirus infections, in particular, are characterized by very high frequencies of PBs during acute infection, suggesting that this cell type has a critical role in the antiviral immune response (
28–30). Unlike other B cell subsets, PBs are antibody-secreting cells and thought to underlie the circulating antibodies during infection (
31). In the long-term, a subset of PBs is retained in the bone marrow as long-lived antibody-secreting cells and generates the circulating antibodies that a subsequent flavivirus will encounter before new antibodies are produced by re-activated B cells (
32). While PBs from flavivirus infections are found to contain both neutralizing and enhancing antibody specificities, some of the most potent flavivirus-neutralizing antibodies were derived from PBs (
33–39). PBs can accumulate mutations to increase in affinity and neutralization potency against an incoming virus, so this represents an adaptable and functional subset of B cells (
34,
40). Because PBs can derive from prior memory B cells, are highly frequent during the pivotal time of acute infection, and can shape the long-lasting and neutralizing antibody compartment, this cell type is one of the key drivers of immune imprinting across multiple infections. Understanding the characteristics of this compartment can explain how immune responses are modulated across multiple flavivirus infections. Indeed, ZIKV-reactive PBs from DENV-immune donors demonstrate more biased clonal repertoires and specificity to previous viruses than those with primary ZIKV infections, suggesting that flavivirus infection history impacts subsequent B cell immunity (
34,
37).
In this study, we sought to understand how prior DENV serotype 3 (DENV-3) infection modulates PB responses in secondary ZIKV infection and distinguish DENV-3-primed cross-reactive signatures from ZIKV-only immunity. We conducted single-cell sequencing of PBs responding to ZIKV infection in order to define specific biases in the repertoire due to differential pre-existing DENV immunity. We hypothesized that cross-reactive antibodies across a heterologous DENV and ZIKV infection would be encoded by distinct PB immunoglobulin variable genes from that of primary or repeat ZIKV infection. We studied immune responses in ZIKV followed by DENV-3 infection because this recapitulates the sequences of flavivirus epidemics in Latin America (
41,
42). We sought to model this sequence of infections with rhesus macaques to allow control of the number of prior exposures and collect blood samples at both baseline and peak PB response for each ZIKV infection. To delineate distinct contributions of prior DENV-3 on ZIKV immunity, we intensively studied the PB responses in a small number of rhesus monkeys with distinct flavivirus exposure status: primary ZIKV infection (
n = 2), sequential ZIKV infection (
n = 2), and a DENV-3 then ZIKV infection (
n = 2). We performed an in-depth PB isolation, phenotyping, and Ig receptor sequencing to compare PB activation, immunoglobulin variable region gene usage, clonal diversity, somatic hypermutation, and ZIKV and DENV-3 reactivity of mAbs derived from isolated single PBs across groups. Our single-cell sequence data from PBs allowed more granular insights on immune imprinting, with the ability to track B cell clones, than cross-reactivity observed in serum antibodies. We found that ZIKV infection after prior DENV demonstrated comparable PB activation levels and formation of ZIKV-neutralizing antibodies but differential IgG isotype usage among ZIKV-reactive PBs, less diversity of clones in the PB repertoire, and lower somatic hypermutation compared with primary and secondary ZIKV-only infections in these macaques. Differences in qualities of PB responses based on infection history supports imprinting from DENV-3 onto ZIKV immunity, and this may have implications for disease outcomes of subsequent infection and vaccination in a setting of multiple flavivirus exposures.
DISCUSSION
In this study, we examined the contribution of pre-existing DENV-3 immunity on early ZIKV B cell immunity by conducting a detailed single-cell genomic analysis of the immunoglobulin variable regions of PBs after serial flavivirus infections. PBs are transiently expanded upon ZIKV and DENV infections and thought to be essential to the formation of protective antiviral immunity (
11,
29,
51–53). Using an Indian rhesus macaque model of flavivirus infections, we generated optimal comparator groups of primary ZIKV and multiple ZIKV challenges that allowed us to specifically detect differences in secondary ZIKV PB responses that may be related to prior DENV-3. While prior studies from our group and others characterized PB response kinetics with controlled series of infections (
26,
44,
54–56), here, we additionally sequenced the B cell receptor variable regions of 363 PBs and characterized 177 derivative monoclonal antibody supernatants from acute ZIKV infection to dissect the signature associated with DENV-3 imprinting onto subsequent PB immunity to ZIKV. We defined the primary B cell variable genes used, as well as differences in PB gene diversity, mutational burden, and isotypes that varied in the acute response to ZIKV in relation to prior infection.
B cell receptor variable gene recombination underlies antigen specificity and subsequent clonal selection into the antiviral immune response (
31). Among ZIKV-reactive PB sequences, we identified several variable genes with human homologs constituting established ZIKV- and flavivirus-reactive specificities. Remarkably, we identified ZIKV-reactive PBs using V
H3-d and Vκ1-e genes. These correspond to the commonly used human V
H3-23 and Vκ1–5 genes and are likely due to convergent gene evolution within a ZIKV-specific B cell response (
34,
57). These sequences may define a common mode of pan-flavivirus recognition, since derived mAbs interact with ZIKV EDIII, Zika whole virion, DENV E protein, and YFV E protein (
9,
34,
46–50,
57). Also, repeated isolations of both V
H3-23 and Vκ1–5 flavivirus-specific mAbs suggest that both of these heavy and light chains are being preferentially selected within a ZIKV immune response. Many of our other sequences matched previously reported human variable genes represented in flavivirus antibodies, including V
H3-48, V
H3-66, V
H4-39, V
H4-59, Vλ5–39, Vκ1–12, Vκ2–28, Vκ3–20, Vλ3–21, and Vλ1–51 (
9,
46–50). This homology demonstrates that our macaque model recapitulated human flavivirus immunoglobulin gene use. Importantly, recurrent isolations of common light chains in the response to ZIKV infections in our data, together with prior studies, reinforce a role for B cell receptor light chains in flavivirus recognition, even though heavy chains are most often studied (
34,
47,
49,
50,
57).
Although we had hypothesized that flavivirus cross-reactivity with and without prior DENV-3 would manifest in distinct PB variable gene signatures in the DENV-3 immune versus ZIKV-only infection groups, we found no clear bias in ZIKV-induced PB variable gene usage. This and prior studies reveal that both ZIKV and DENV infections are associated with some common PB genes within families V
H3 and V
H4 (
9,
46,
48,
49). A few ZIKV-reactive PB genes may be associated with immune history, such as V
H3-aa and V
H3-ag (human gene: V
H3-13 and V
H3-66), which was found only in DENV-immune ZIKV responses, whereas V
H3-j (human gene: V
H3-49) was found only in ZIKV-only responses. While it remains possible that biases in gene use exist by exact flavivirus infection history, we were not able to test this with only 363 singly sequenced cells from our data set due to the enormous diversity of variable gene usage we uncovered among the acute ZIKV infection PB response, although, interestingly, V
H3-49 was found to be DENV and yellow fever virus binding and V
H3-66 neutralized both ZIKV and DENV-3 in this study (
47,
48). The broad cross-reactivity associated with these genes cumulatively across studies suggests that these Ig genes may be recruited for antibodies that target highly conserved epitope studies (
47,
48). Thus, different immune histories may provide a host with different evolutionary starting points to generate a flavivirus cross-reactive B cell repertoire.
The diversity of variable genes in the transient PB population is related to B cell selection, where low diversity indicates few distinct B cell clones present in a population and suggests preferential selection for these clones (
58). Among the ZIKV-reactive PBs, we identified the highest V
H and V
L diversity levels upon primary ZIKV infection and lower levels in both homologous and heterologous secondary ZIKV responses. This transition from primary to secondary ZIKV-ZIKV is expected as anti-ZIKV B cells established in the primary infection can be preferentially selected into the secondary immune response, expand to occupy more clonal space, and thereby decrease diversity in PBs. A similar pattern of decreased diversity among circulating PBs was noted in secondary human ZIKV and DENV infections (
34,
46,
57). We also hypothesized that a primary DENV-3 infection would be associated with fewer or weaker ZIKV-reactive B cells than a primary ZIKV infection, and so, there may be fewer clones to select into a secondary DENV-3-ZIKV PB response than in ZIKV-ZIKV infection. In contrast, we found that the diversity among ZIKV-reactive PBs in the DENV-3-ZIKV group was on par with ZIKV-ZIKV and significantly lower than primary ZIKV. This interesting pattern is compatible with immune imprinting and suggests that a recall response from primary DENV-3 infection may contribute to the secondary ZIKV PB response. A similar pattern has also been reported in five humans with acute ZIKV infection and pre-existing DENV immunity (
34,
40). Moreover, boosting of plasma DENV-3 binding and neutralizing antibodies upon ZIKV infection in these macaques supports elicitation of a recall response in these two animals. It is possible that DENV-3 recall supports ZIKV-neutralizing antibodies since the strongest ZIKV-binding mAbs from the DENV-3-ZIKV group also cross-reacted with DENV-3, suggesting a recall origin for these specificities. Whereas mAbs from the DENV-3-ZIKV infection group that did not cross-react with DENV-3 showed lower levels of ZIKV binding, and these may be elicited
de novo. Epidemiologically, prior DENV immunity is associated with milder subsequent ZIKV (
59,
60), and our work suggests this may, in part, be due to immune focusing toward highly ZIKV-binding and neutralizing antibody specificities.
Somatic mutations in B cell variable regions can further increase the affinity and potency of the antibody response (
31). We observed moderate to high levels of somatic hypermutation in the immunoglobulin heavy and light variable regions of ZIKV-reactive PBs that mirrored prior estimates from ZIKV-infected patients (
34,
40,
61,
62). Due to conflicting evidence on prior DENV infections, it is unclear how SHM of PBs evaluated in our study compare with that of DENV infection (
40,
48,
63). One intriguing observation is that boosting of SHM in secondary ZIKV-ZIKV infection occurred upon challenge but in the absence of measurable viremia, supporting the potential for asymptomatic/subclinical infections in flavivirus co-endemic areas to generate more mature B cells over time (
64). Interestingly, ZIKV-ZIKV infection-induced PBs acquired significantly higher SHM than DENV-3-ZIKV infection-induced PBs. Thus, the identical ZIKV rechallenge is associated with more B cell maturation within a limited clonal diversity than DENV-3-ZIKV infection. It is possible that a ZIKV-ZIKV rechallenge may be better at promoting immune focusing than DENV-3-ZIKV because it is an identical repeat exposure. This phenomenon of immune focusing upon identical re-exposure to antigens was also observed with the SARS-CoV-2 mRNA vaccine booster series (
65). Time since last infection can also modulate the extent of boosting in SHM. Recent studies indicate that virus-reactive germinal centers can persist up to 3–5 months, suggesting that a later infection, beyond the timeframe of our study, may also result in a bigger boost than earlier infections (
66,
67). Thus, the timing between viral challenges and doses used in our study may limit our inferences. It is possible that a challenge that is administered at a later timepoint may differentially modulate the PB response. Further studies should continue to delineate the contributions of sequential related infections on B cell diversity and mutations.
Finally, the distribution of antiviral immunoglobulin isotypes impacts antibody functional profile and immune protection (
12). For example, pre-existing antiviral antibodies of the IgG1, IgG4, IgA2, and IgM isotypes are correlated with protection against subsequent symptomatic DENV-3 infection (
12). In our ZIKV-only challenge groups, ZIKV-reactive specificities arose from IgM and IgA PBs, whereas prior DENV-3 infection was associated with a different profile such that ZIKV-reactive specificities arose from the IgG compartment. IgG and early IgM antibodies are known to potently neutralize the virus (
49,
57,
68,
69), whereas cross-reactive IgG can also enhance viral replication (
49,
70–72). Intriguingly, we found a large rise of IgA+ PBs in all primary and secondary ZIKV infections, which has also been observed for DENV and SARS-CoV-2 infections, and influenza vaccination (
32,
67,
73,
74). While Waickman et al. reported higher antiviral IgA-expressing PBs in primary rather than secondary DENV infection, we find similarly high levels of IgA-expressing PBs in primary and secondary ZIKV infection (
73). Anti-DENV IgA were found to be effective disruptors of IgG-mediated DENV enhancement
in vitro and were broadly neutralizing (
71,
73,
75). While there is evidence on the value of IgA antibodies in DENV infections, the role for IgA in ZIKV infection remains unclear.
Small sample sizes, the number of macaques, and cryopreservation of fragile PBs were limitations of this study. Given two animals per group, the role of inter-individual variability and timing of past exposure could not be deconvoluted from infection history, our primary analytic variable. Thus, our inferences are based on associations and delineating causal relationships will require further investigation.
In summary, we found ZIKV infection after DENV-3 infection was associated with early formation of high-magnitude ZIKV-binding and neutralizing antibodies and PB responses, despite evidence of immune imprinting by recall responses at both the antibody and B cell levels. Indeed, DENV-3-ZIKV and ZIKV-ZIKV infection-induced PBs demonstrate the use of similar immunoglobulin variable genes, indicating common approaches to ZIKV recognition despite differences in prior infection. Also, acute PB responses in both secondary DENV-3-ZIKV and ZIKV-ZIKV infection are characterized by significantly lower clonal diversity than primary ZIKV infection, whereas multiple ZIKV exposures are associated with more B cell maturation via SHM. Interestingly, a higher proportion of IgA isotype PBs was found in ZIKV-only immune histories, where ZIKV-reactive IgA PBs were more somatically mutated than non-IgA ZIKV-reactive PBs. It is possible that IgA antibodies and B cells may differentially modulate anti-ZIKV immune functions based on prior exposure. Overall, immune imprinting by DENV was associated with a productive ZIKV immune response and was characterized by many shared B cell receptor genes but qualitatively distinct mutational and isotype distribution in the acute B cell response.