CD19(+) B cells confer protection against experimental cerebral malaria in semi-immune rodent model

Current understanding of the immune potential of B-cell subsets in malarial pathogenesis

In the past several decades, our understanding of how B cells are generated and what function they perform has continued to advance. It is widely accepted that B-cell subsets play a critical role in mediating immune response. Surprisingly, human and murine malarial infections cause major alterations in the composition of B-cell subsets in both the spleen and periphery. Multiple B-cell subsets are well characterized in murine models following primary and secondary infection, although in human malarial infection, these subsets are not well defined. Furthermore, a rare known function of B cells includes the potential role of regulating the activities of other cells in the body as regulatory cells. Plasmodium infection strongly alters the frequency of these regulatory B cells indicating the immunoregulatory function of B cells in malarial. It is important to note that these subsets, taken together, form the cellular basis of humoral immune responses, allowing protection against a wide array of Plasmodium antigens to be achieved. However, it remains a challenge and an important area of investigation to understand how these B-cell subsets work together to provide protection against Plasmodium infection.

Immunosuppression in Malaria: Do Plasmodium falciparum Parasites Hijack the Host?

Malaria reflects not only a state of immune activation, but also a state of general immune defect or immunosuppression, of complex etiology that can last longer than the actual episode. Inhabitants of malaria-endemic regions with lifelong exposure to the parasite show an exhausted or immune regulatory profile compared to non- or minimally exposed subjects. Several studies and experiments to identify and characterize the cause of this malaria-related immunosuppression have shown that malaria suppresses humoral and cellular responses to both homologous (Plasmodium) and heterologous antigens (e.g., vaccines). However, neither the underlying mechanisms nor the relative involvement of different types of immune cells in immunosuppression during malaria is well understood. Moreover, the implication of the parasite during the different stages of the modulation of immunity has not been addressed in detail. There is growing evidence of a role of immune regulators and cellular components in malaria that may lead to immunosuppression that needs further research. In this review, we summarize the current evidence on how malaria parasites may directly and indirectly induce immunosuppression and investigate the potential role of specific cell types, effector molecules and other immunoregulatory factors.

Memory CD8+ T cells exhibit tissue imprinting and non-stable exposure-dependent reactivation characteristics following blood-stage Plasmodium berghei ANKA infections

Experimental cerebral malaria (ECM) is a severe complication of Plasmodium berghei ANKA (PbA) infection in mice, characterized by CD8⁺ T‐cell accumulation within the brain. Whilst the dynamics of CD8⁺ T‐cell activation and migration during extant primary PbA infection have been extensively researched, the fate of the parasite‐specific CD8⁺ T cells upon resolution of ECM is not understood. In this study, we show that memory OT‐I cells persist systemically within the spleen, lung and brain following recovery from ECM after primary PbA‐OVA infection. Whereas memory OT‐I cells within the spleen and lung exhibited canonical central memory (Tcm) and effector memory (Tem) phenotypes, respectively, memory OT‐I cells within the brain post‐PbA‐OVA infection displayed an enriched CD69⁺CD103⁻ profile and expressed low levels of T‐bet. OT‐I cells within the brain were excluded from short‐term intravascular antibody labelling but were targeted effectively by longer‐term systemically administered antibodies. Thus, the memory OT‐I cells were extravascular within the brain post‐ECM but were potentially not resident memory cells. Importantly, whilst memory OT‐I cells exhibited strong reactivation during secondary PbA‐OVA infection, preventing activation of new primary effector T cells, they had dampened reactivation during a fourth PbA‐OVA infection. Overall, our results demonstrate that memory CD8⁺ T cells are systemically distributed but exhibit a unique phenotype within the brain post‐ECM, and that their reactivation characteristics are shaped by infection history. Our results raise important questions regarding the role of distinct memory CD8⁺ T‐cell populations within the brain and other tissues during repeat Plasmodium infections.

Higher CD19+CD25+ Bregs are independently associated with better graft function in renal transplant recipients

Background

The Identification of B cell subsets with regulatory functions might open the way to new therapeutic strategies in the field of transplantation, which aim to reduce the dose of immunosuppressive drugs and prolong the graft survival. CD25 was proposed as a marker of a B-cell subset with an immunosuppressive action termed Bregs. The effect of CD19 + CD25 + Bregs on graft function in renal transplant recipients has not yet been elucidated. We investigated a potential impact of CD19 + CD25 + Bregs on renal graft function as well as a possible interaction of CD19 + CD25 + Bregs with peripheral Tregs in healthy controls, end-stage kidney disease patients (ESKD), and renal transplant recipients. Moreover, we aimed to investigate the association of CD19 + CD25 + Bregs with serum IL-10, TGF-ß1, and IFN-γ in the same study groups.

Method

Thirty-one healthy controls, ninety renal transplant recipients, and eighteen ESKD patients were enrolled. We evaluated the CD19 + CD25 + Bregs and Treg absolute counts. Next, we investigated CD19 + CD25 + Bregs as predictors of good graft function in multiple regression and ROC analyses. Finally, we evaluated the association between CD19 + CD25+ Bregs and serum IL-10, TGF-ß, and IFN-γ.

Results

ESKD patients and renal transplant recipients showed lower counts of CD19 + CD25+ Bregs compared to healthy controls (p < 0.001). Higher CD19 + CD25+ Breg counts were independently associated with a better GFR in renal transplant recipients (unstandardized B coefficient = 9, p = 0.02). In these patients, higher CD19 + CD25+ Bregs were independently associated with higher Treg counts (unstandardized B = 2.8, p = 0.004). In ROC analysis, cut-offs for CD19 + CD25 + Breg counts and serum TGF-ß1 of 0.12 cell/μl and 19,635.4 pg/ml, respectively, were shown to provide a good sensitivity and specificity in identifying GFR ≥ 30 ml/min (AUC = 0.67, sensitivity 77%, specificity 43%; AUC = 0.65, sensitivity 81%, specificity 50%, respectively). Finally, a significant positive association between CD19 + CD25+ Bregs and TGF-ß1 was shown in renal transplant recipients (r = 0.255, p = 0.015).

Conclusions

Our findings indicate that higher counts of CD19 + CD25+ Bregs are independently associated with better renal function and higher absolute Treg counts in renal transplant recipients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-021-02374-2.

Infection-Induced Resistance to Experimental Cerebral Malaria Is Dependent Upon Secreted Antibody-Mediated Inhibition of Pathogenic CD8+ T Cell Responses

Cerebral malaria (CM) is one of the most severe complications of Plasmodium falciparum infection. There is evidence that repeated parasite exposure promotes resistance against CM. However, the immunological basis of this infection-induced resistance remains poorly understood. Here, utilizing the Plasmodium berghei ANKA (PbA) model of experimental cerebral malaria (ECM), we show that three rounds of infection and drug-cure protects against the development of ECM during a subsequent fourth (4X) infection. Exposure-induced resistance was associated with specific suppression of CD8+ T cell activation and CTL-related pathways, which corresponded with the development of heterogeneous atypical B cell populations as well as the gradual infection-induced generation and maintenance of high levels of anti-parasite IgG. Mechanistically, transfer of high-titer anti-parasite IgG did not protect 1X infected mice against ECM and depletion of atypical and regulatory B cells during 4X infection failed to abrogate infection-induced resistance to ECM. However, IgMi mice that were unable to produce secreted antibody, or undergo class switching, during the repeated rounds of infection failed to develop resistance against ECM. The failure of infection-induced protection in IgMi mice was associated with impaired development of atypical B cell populations and the inability to suppress pathogenic CD8+ T cell responses. Our results, therefore, suggest the importance of anti-parasite antibody responses, gradually acquired, and maintained through repeated Plasmodium infections, for modulating the B cell compartment and eventually suppressing memory CD8+ T cell reactivation to establish infection-induced resistance to ECM.

TACI Contributes to Plasmodium yoelii Host Resistance by Controlling T Follicular Helper Cell Response and Germinal Center Formation

The delay in parasite-specific B cell development leaves people in malaria endemic areas vulnerable to repeated Plasmodium infections. Here, we investigated the role of transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), a molecule involved in the generation of antigen-specific antibody secreting cells, in host response to non-lethal Plasmodium yoelii infection. We found that TACI deficiency not only resulted in higher peak parasitemia levels in P. yoelii challenged mice, but also led to a delay in parasite clearance and anti-P. yoelii Merozoite Surface Protein 1(C-terminal 19-kDa fragment [rMSP-1₁₉]) protein and anti-rMSP-1₁₉ and anti-P. yoelii IgG antibody development. There was also a delay in the generation of splenic high affinity antibody secreting cells that recognize rMSP-1₁₉ protein as compared to wild-type mice. Interestingly, coinciding with the delay in parasite clearance there was a delay in the resolution of T follicular helper (T_FH) cell and germinal center (GC) B cell responses in TACI -/- mice. The persistence of T_FH and GC B cells is likely a result of enhanced interaction between T_FH and GC B cells because inducible costimulator ligand (ICOSL) expression was significantly higher on TACI -/- GC B cells than wild-type cells. The difference in the kinetics of GC reaction appeared to also impact the emergence of plasma cells (PC) because there was a delay in the generation of TACI -/- mice PC. Nevertheless, following the recovery from P. yoelii infection, TACI -/- and wild-type mice were both protected from a rechallenge infection. Establishment of protective B cell response was responsible for the resolution of parasitemia because B cells purified from recovered TACI -/- or wild-type mice were equally protective when introduced to naïve wild-type mice prior to P. yoelii challenge. Thus, despite the increased susceptibility of TACI -/- mice to P. yoelii infection and a delay in the development of protective antibody levels, TACI -/- mice are able to clear the infection and resist rechallenge infection.

Personal history of infections and immunotherapy: Unexpected links and possible therapeutic opportunities

The recent breakthroughs in the understanding of tumor immune biology have given rise to a new generation of immunotherapies, harnessing the immune system to eliminate tumors. As the typology and frequency of encountered infections are susceptible to shape the immune system, it could also impact the efficiency of immunotherapy. In this review, we report evidences for an indirect link between personal history of infection and different strategies of immunotherapy. In the current context of interest rise for personalized medicine, we discuss the potential medical applications of considering personal history of infection to design immunotherapeutic strategies.

Potential Role for Regulatory B Cells as a Major Source of Interleukin-10 in Spleen from Plasmodium chabaudi-Infected Mice

Interleukin-10 (IL-10)-producing regulatory B (Breg) cells were found to be induced in a variety of infectious diseases. However, its importance in the regulation of immune response to malaria is still unclear. Here, we investigated the dynamics, phenotype, and function of Breg cells using Plasmodium chabaudi chabaudi AS-infected C57BL/6 and BALB/c mice. BALB/c mice were more susceptible to infection and had a stronger IL-10 response in spleen than C57BL/6 mice. Analysis of the surface markers of IL-10-producing cells with flow cytometry showed that CD19⁺ B cells were one of the primary IL-10-producing populations in P. c. chabaudi AS-infected C57BL/6 and BALB/c mice, especially in the latter one. The Breg cells had a heterogeneous phenotype which shifted during infection. The well-established Breg subset, CD19⁺ CD5⁺ CD1d^hi cells, accounted for less than 20% of IL-10-producing B cells in both strains during the course of infection. Most Breg cells were IgG⁺ and CD138^- from day 0 to day 8 postinfection. Adoptive transfer of Breg cells to C57BL/6 mice infected with P. c. chabaudi AS led to a transient increase of parasitemia without an impact on survival rate. Our finding reveals that B cells play an active and important regulatory role in addition to mediating humoral immunity in immune response against malaria, which should be paid more attention in developing therapeutic or vaccine strategies against malaria involving stimulation of B cells.

What do we know about the role of regulatory B cells (Breg) during the course of infection of two major parasitic diseases, malaria and leishmaniasis?

Parasitic diseases, such as malaria and leishmaniasis, are relevant public health problems worldwide. For both diseases, the alarming number of clinical cases and deaths reported annually has justified the incentives directed to better understanding of host's factors associated with susceptibility to infection or protection. In this context, over recent years, some studies have given special attention to B lymphocytes with a regulator phenotype, known as Breg cells. Essentially important in the maintenance of immunological tolerance, especially in autoimmune disease models such as rheumatoid arthritis and experimentally induced autoimmune encephalomyelitis, the function of these lymphocytes has so far been poorly explored during the course of diseases caused by parasites. As the activation of Breg cells has been proposed as a possible therapeutic or vaccine strategy against several diseases, here we reviewed studies focused on understanding the relation of parasite and Breg cells in malaria and leishmaniasis, and the possible implications of these strategies in the course of both infections.

Tacrolimus prevents murine cerebral malaria

Tacrolimus and mycophenolate mofetil are immunosuppressants frequently used in human organ transplantation. Tacrolimus is also reported to inhibit Plasmodium falciparum growth in vitro. Here, we report that tacrolimus prevented the death from cerebral malaria of Plasmodium berghei ANKA-infected C57BL/6J mice, but not their death from malaria due to the high parasitaemia and severe anaemia. The mycophenolate mofetil-treated mice showed higher mortality from cerebral malaria and succumbed to malaria earlier than tacrolimus-treated littermates. Tacrolimus attenuated the infiltration of mononuclear cells including pathogenic CD8⁺ T cells into the brain. It appears to prevent murine cerebral malaria through the inhibition of cerebral infiltration of CD8⁺ T cells.

Parasite-Specific CD4+ IFN-γ+ IL-10+ T Cells Distribute within Both Lymphoid and Nonlymphoid Compartments and Are Controlled Systemically by Interleukin-27 and ICOS during Blood-Stage Malaria Infection

Immune-mediated pathology in interleukin-10 (IL-10)-deficient mice during blood-stage malaria infection typically manifests in nonlymphoid organs, such as the liver and lung. Thus, it is critical to define the cellular sources of IL-10 in these sensitive nonlymphoid compartments during infection. Moreover, it is important to determine if IL-10 production is controlled through conserved or disparate molecular programs in distinct anatomical locations during malaria infection, as this may enable spatiotemporal tuning of the regulatory immune response. In this study, using dual gamma interferon (IFN-γ)–yellow fluorescent protein (YFP) and IL-10–green fluorescent protein (GFP) reporter mice, we show that CD4⁺ YFP⁺ T cells are the major source of IL-10 in both lymphoid and nonlymphoid compartments throughout the course of blood-stage Plasmodium yoelii infection. Mature splenic CD4⁺ YFP⁺ GFP⁺ T cells, which preferentially expressed high levels of CCR5, were capable of migrating to and seeding the nonlymphoid tissues, indicating that the systemically distributed host-protective cells have a common developmental history. Despite exhibiting comparable phenotypes, CD4⁺ YFP⁺ GFP⁺ T cells from the liver and lung produced significantly larger quantities of IL-10 than their splenic counterparts, showing that the CD4⁺ YFP⁺ GFP⁺ T cells exert graded functions in distinct tissue locations during infection. Unexpectedly, given the unique environmental conditions within discrete nonlymphoid and lymphoid organs, we show that IL-10 production by CD4⁺ YFP⁺ T cells is controlled systemically during malaria infection through IL-27 receptor signaling that is supported after CD4⁺ T cell priming by ICOS signaling. The results in this study substantially improve our understanding of the systemic IL-10 response to malaria infection, particularly within sensitive nonlymphoid organs.

Splenic CD11c+ cells derived from semi-immune mice protect naïve mice against experimental cerebral malaria

Background

Immunity to malaria requires innate, adaptive immune responses and Plasmodium-specific memory cells. Previously, mice semi-immune to malaria was developed. Three cycles of infection and cure (‘three-cure’) were required to protect mice against Plasmodium berghei (ANKA strain) infection.

Methods

C57BL/6 J mice underwent three cycles of P. berghei infection and drug-cure to become semi-immune. The spleens of infected semi-immune mice were collected for flow cytometry analysis. CD11c(+) cells of semi-immune mice were isolated and transferred into naïve mice which were subsequently challenged and followed up by survival and parasitaemia.

Results

The percentages of splenic CD4(+) and CD11c(+) cells were increased in semi-immune mice on day 7 post-infection. The proportion and number of B220(+)CD11c(+)low cells (plasmacytoid dendritic cells, DCs) was higher in semi-immune, three-cure mice than in their naïve littermates on day 7 post-infection (2.6 vs 1.1% and 491,031 vs 149,699, respectively). In adoptive transfer experiment, three months after the third cured P. berghei infection, splenic CD11c(+) DCs of non-infected, semi-immune, three-cure mice slowed Plasmodium proliferation and decreased the death rate due to neurological pathology in recipient mice. In addition, anti-P. berghei IgG1 level was higher in mice transferred with CD11c(+) cells of semi-immune, three-cure mice than mice transferred with CD11c(+) cells of naïve counterparts.

Conclusion

CD11c(+) cells of semi-immune mice protect against experimental cerebral malaria three months after the third cured malaria, potentially through protective plasmacytoid DCs and enhanced production of malaria-specific antibody.