Developing a vaccine against HIV-1 and understanding why the neutralizing Ab response is globally inefficient remains a challenge. Defaults in the HIV-specific Ab response were widely thought to result from a loss in CD4+ T-cells, but recent in-depth examinations of the B-cell population during pathogenic and non-pathogenic HIV/SIV infection have challenged this idea. These pioneering studies have largely contributed to change our global understanding of the role of B-cells.
B-cells during HIV/SIV infection
B-cell dysfunctions are now considered to be a central feature of HIV infection and an important pathogenic mechanism [
69-
71]. Although B-cell hyper-activation, including centro-follicular hyperplasia, and hypergammaglobulinemia, with IgG1 being the most deregulated, were among the first symptoms described in HIV-infected patients [
72-
74], the role of B-cells in HIV/SIV progression has been largely underappreciated until recently. One extremely puzzling issue in HIV infection is the global inefficiency of the HIV-induced Ab response. Cumulative data reveal that circulating virus-specific Abs are detectable by one month of infection, whereas neutralizing Abs are undetectable until after 3 months. Broadly neutralizing Abs generally develop after one or two years and in only 10–30% of untreated HIV-infected patients [
75]. Most neutralizing Abs are directed against HIV gp120 or gp41 proteins or their binding sites on CD4, CCR5, or CXCR4, and have features of poly-reactive or self-reactive Abs [
76]. Along with the virus-specific Ab response, the humoral response to non-HIV Ags is strongly impaired, resulting in a decreased response to natural or vaccine TI and TD Ags as early as during the acute phase of infection [
71,
77]. Together, these data suggest that both the innate (TI) and virus-specific (TD) arms of the Ab response are impaired during HIV infection.
Chronically HIV-infected patients are reported to experience a loss in circulating MZ-like B-cells, associated with an impaired response to pneumococcal Ags [
77,
78]. Similarly, following infection, primary SIV-infected macaques have reduced proportions of MZ B-cells, not only in blood but also in spleen and peripheral lymph nodes [
79]. Additionally, increases in circulating IgM and IgG levels and in PC numbers were observed in the spleen MZ of these animals from two weeks post-infection. Thus, virus-activated MZ B-cells likely differentiate into PC. This idea is consistent with a report showing that gp120-activated MZ-like B-cells rapidly produce IgG and IgA [
80]. However, the most striking effect of HIV infection occurs within the MemB pool. Resting MemB constitute the predominant fraction of blood MemB in healthy donors, with low percentages of activated and atypical MemB [
81]. In contrast, there is a paucity of resting MemB while both activated and atypical MemB are over-represented in the blood of chronically HIV-infected patients [
70]. A similar decrease in resting MemB has been reported during pathogenic SIV infection [
53,
79,
82], and this loss is concomitant with BAFF overproduction during the acute phase [
53].
In chronically HIV-infected patients, atypical MemB are exhausted B-cells that express FcRL4 and other inhibitory receptors and are unresponsive to BCR triggering [
50]. These cells, however, are highly responsive to TLR9 ligands and, therefore, could play a role in Ab or cytokine production. FcRL4 expression appears to protect MemB from the deleterious effects of chronic infection or inflammation [
51]. Within the atypical MemB pool, HIV-specific Abs are enriched, and their production might be further enhanced by treatment with short-interfering RNA targeting FcRL4 or SIGLEC-6 [
50,
83]. FcRL4 expression and TGFβ1 production are induced by the binding of recombinant gp120 to the α4β7 integrin expressed by naïve B-cells [
84]. Co-culture of B-cells with CD4
+ T-cells from HIV-infected donors similarly up-regulates B-cell FcRL4 expression. Interactions between gp120 and α4β7 also reduce B-cell proliferative responses and CD80 expression [
84]. The latter is consistent with our previous data showing decreased CD80, but not CD86, expression in GC B-cells from chronically HIV-infected patients [
85]. Thus HIV-1 might impair both the BCR responses and co-stimulation abilities of B-cells, at least during the chronic phase of infection. Moreover, X4 gp120 proteins strongly reduce B-cell chemotaxis to not only CXCL12 but also to CCL20 and CCL21 by cross-desensitization of CCR6 and CCR7. Additionally, they induce CD62L cleavage and enhance MemB CD95 expression [
86]. In summary, HIV has developed various envelope-based strategies to subvert B-cell responses, survival, and trafficking.
A key checkpoint for adaptive B-cell responses is the GC reaction leading to the generation of MemB and long-lived PC precursors. Although GC hyperplasia during pathogenic HIV/SIV infection was described long ago [
85,
87,
88], the precise impact of the virus on GC B-cells remains elusive. We previously described the well-conserved organization and polarization of GC from the in splenic, nodular and intestinal follicles during primary SIV infection [
53,
79]. Levesque
et al. observed GC fragmentation in primary HIV-infected patients [
89], but generally GC involution is more frequent during the chronic and advanced phases of HIV infection when CXCR4 variants are present [
85]. Similarly, early GC disruption occurs after SIV infection of Indian rhesus macaques, a model of rapid disease progression, [
90] but not in the more typical models using cynomolgus or Chinese rhesus macaques [
53,
91].
Recent progress on the characterization of T
FH cells has clarified some points. First, circulating or nodular T
FH cells are infected by HIV/SIV similarly to, or even more strongly than other CD4
+ T-cells, but survive longer despite continuous exposure to virus [
53,
92-
94]. Second, during the acute phase of infection T
FH cells are moderately expanded in most individuals, with a correlation between tissue viral load and percentages of T
FH cells [
95]. In contrast, chronically HIV-infected individuals and SIV-infected animals have strong inter-individual variation in their percentages of T
FH cells [
92-
94]. However, conflicting results have been reported regarding the correlation between viral load and proportions of T
FH during the chronic phase of infection [
92,
93]. Based on the proportions of CD4
+CD45RO
+ or CD4
+PD1
hi T cells in GC, it was possible to correlate T
FH and GC hyperplasia in SIV-infected macaques and in the lymph nodes of chronically HIV-infected patients by
in situ analysis [
53,
91,
93]. In summary, during HIV/SIV infection T
FH cells are expanded and GCs are correctly polarized but the virus-specific response is delayed, and when it occurs, it provides relatively inefficient protection.
These paradoxical findings suggest that more subtle dysfunctions of GC B-cells, T
FH cells, or of their dialog occur during HIV infection and impair either the generation (within GC) or the survival and trafficking of effector B-cells (MemB or PC). The production of MemB with “alternate” phenotypes is consistent with a dysfunction of GC B-cells but might coexist with other impairments. Given that the virus is able to replicate within T
FH cells, gp120, Tat, and Nef proteins might be locally over-produced and interfere with the GC reaction. Indeed, Nef was shown to affect Ig class switching [
96], and soluble Tat selectively increases CD40-mediated proliferation of GC B-cells [
97]. In-depth phenotypic, molecular, and functional analyses of B-cell and T-cell subsets within GC and at the follicular border during the priming phase are required for a better understanding of the HIV-induced defaults that cause inappropriate Ab responses.
In this already complex situation, a new B-cell subset with regulatory functions has been recently identified. This population with a CD19
+CD38
hiCD24
hiPD-L1
+ (CD27
−) phenotype spontaneously secretes IL10 and inhibits CD8
+ T-cell proliferation and the HIV-specific cytotoxic response in antiretroviral-treated or untreated HIV-infected patients [
9]. Besides IL10, PD-L1/PD1 interactions are assumed to critically contribute to CD8
+ T-cell exhaustion. Patients with advanced HIV-disease also have increased proportions of circulating CD10
+ immature-transitional B-cells [
98]. Because IL7 and BAFF plasma levels were elevated in these patients [
98,
99], bone marrow dysfunctions and/or lymphopenia are thought to induce CD10
+ B-cell mobilization into the periphery. Moreover, our data suggest that CD10
+CD38
+SIgD
+ B-cells, which are more numerous in HIV-infected patients with a high Epstein-Barr virus (EBV) viral load and a strong depletion of resting MemB, might constitute an alternate EBV reservoir [
100]. Because EBV
+ B-cell lymphomas occur with a higher incidence in HIV-infected individuals than in the general population [
101], the contribution of these CD10
+ B-cells should be further examined.
B-cells during hepatitis infection
Similarly to infection with HIV, infection with HBV or hepatitis C virus (HCV) is associated with polyclonal B-cell activation. When produced during the acute phase of infection, neutralizing Abs are associated with viral clearance [
102]; unfortunately they frequently develop only during the chronic phase [
103]. In chronically HCV-infected patients, B-cell dysfunction is reflected by IgG1 restriction, with low-titer and delayed-onset Ab responses [
104]. Loss in resting MemB was associated with increased proportions of atypical MemB in HCV-infected patients, regardless of cirrhosis or hepatocellular carcinoma. This increase is likely present as early as during the acute phase of HCV infection. These atypical MemB are hypo-proliferative in response to CD40 or BCR stimulation but produce high amounts of IgG [
105,
106]. Increased MemB IgG production was observed in chronically HBV- and HCV-infected patients [
107]. HCV is the only hepatitis infection model in which B-cell infection by particular virus quasi-species has been strongly demonstrated [
108] and shown to be important for disease outcome [
109]. In chronically HCV-infected patients, elevated levels of serum BAFF have been associated with autoimmunity [
110]
HBV core Ag has the unique capacity to stimulate BCR in a non-Ag specific manner leading to sustained B-cell activation in chronically HBV-infected patients [
107,
111]. Although an extensive phenotypic and functional analysis of B-cells in HBV-infected patients is still lacking, Das
et al. recently identified a unique subset of CD38
hiCD24
hiCD27
− B-regs, whose frequency correlates with spontaneous flares of liver disease, viral load, and serum IL10 levels. This B-cell population inhibits virus-specific CD8
+ T-cell responses, but dampens liver inflammation through IL10 production [
10].