Altered T cell memory and effector cell development in chronic lymphatic filarial infection that is independent of persistent parasite antigen

Adoptive Transfer of Immune Cells Into RAG2IL-2Rγ-Deficient Mice During Litomosoides sigmodontis Infection: A Novel Approach to Investigate Filarial-Specific Immune Responses

Despite long-term mass drug administration programmes, approximately 220 million people are still infected with filariae in endemic regions. Several research studies have characterized host immune responses but a major obstacle for research on human filariae has been the inability to obtain adult worms which in turn has hindered analysis on infection kinetics and immune signalling. Although the Litomosoides sigmodontis filarial mouse model is well-established, the complex immunological mechanisms associated with filarial control and disease progression remain unclear and translation to human infections is difficult, especially since human filarial infections in rodents are limited. To overcome these obstacles, we performed adoptive immune cell transfer experiments into RAG2IL-2Rγ-deficient C57BL/6 mice. These mice lack T, B and natural killer cells and are susceptible to infection with the human filaria Loa loa. In this study, we revealed a long-term release of L. sigmodontis offspring (microfilariae) in RAG2IL-2Rγ-deficient C57BL/6 mice, which contrasts to C57BL/6 mice which normally eliminate the parasites before patency. We further showed that CD4⁺ T cells isolated from acute L. sigmodontis-infected C57BL/6 donor mice or mice that already cleared the infection were able to eliminate the parasite and prevent inflammation at the site of infection. In addition, the clearance of the parasites was associated with Th17 polarization of the CD4⁺ T cells. Consequently, adoptive transfer of immune cell subsets into RAG2IL-2Rγ-deficient C57BL/6 mice will provide an optimal platform to decipher characteristics of distinct immune cells that are crucial for the immunity against rodent and human filarial infections and moreover, might be useful for preclinical research, especially about the efficacy of macrofilaricidal drugs.

Cytotoxic T-Lymphocyte-Associated Antigen 4 (CTLA-4)- and Programmed Death 1 (PD-1)-Mediated Regulation of Monofunctional and Dual Functional CD4+ and CD8+ T-Cell Responses in a Chronic Helminth Infection

Chronic helminth infections are known to be associated with the modulation of antigen-specific T-cell responses. Strongyloides stercoralis infection is characterized by the downmodulation of antigen-specific Th1 and Th17 responses and the upregulation of Th2 and Th9 responses. Immune homeostasis is partially maintained by negative regulators of T-cell activation, cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) and programmed death 1 (PD-1), which dampen effector responses during chronic infections. However, their roles in S. stercoralis infection are yet to be defined. Therefore, we sought to determine the role of CTLA-4 and PD-1 in regulating CD4⁺ and CD8⁺ T-cell responses and examined the frequencies of monofunctional and dual functional Th1/T cytotoxic type 1 (Tc1), Th17/Tc17, Th2/Tc2, and Th9/Tc9 cells in S. stercoralis infection in 15 infected individuals stimulated with parasite antigen following CTLA-4 or PD-1 blockade. Our data reveal that CTLA-4 or PD-1 blockade results in significantly enhanced frequencies of monofunctional and dual functional Th1/Tc1 and Th17/Tc17 cells and, in contrast, diminishes the frequencies of monofunctional and dual functional Th2/Tc2 and Th9/Tc9 cells with parasite antigen stimulation in whole-blood cultures. Thus, we demonstrate that CTLA-4 and PD-1 limit the induction of particular T-cell subsets in S. stercoralis infection, which suggests the importance of CTLA-4 and PD-1 in immune modulation in a chronic helminth infection.

Characterization of memory T cells in individuals resistant to Schistosoma mansoni infection

Schistosomiasis affects about 240 million people worldwide and is estimated that about 700 million people live in areas at risk of infection. In the context of immune response associated with infection by Schistosoma mansoni, the role of memory T cells is not well understood.

AIM

To evaluate the frequency of memory CD4⁺ and CD8⁺ T cells from individuals resistant and susceptible to Schistosoma mansoni infection.

METHODS AND RESULTS

We selected individuals with low (resistant) and high (susceptible) parasite burden using databases generated during previous studies carried out in the same endemic area. The cell surface markers were performed using flow cytometry. In this study, the resistant individuals showed an increase in the CD4⁺ memory T-cell pool associated with an increase in the central memory cell (TCM) and a decrease in the effector memory cell (T_EM ). Individuals susceptible to infection had higher frequencies of effector memory cells compared to resistant individuals.

CONCLUSIONS

These data suggest that resistance to S mansoni infection may be associated with an increase in the number of CD4⁺ memory T cells and susceptibility to infection is associated with a decrease in the central memory cell as well as high proportions of effector memory cells.

Altered levels of memory T cell subsets and common γc cytokines in Strongyloides stercoralis infection and partial reversal following anthelmintic treatment

BACKGROUND

CD4+ and CD8+ T cells are central players in immunity to helminth infections. However, the role of T cell subsets in human helminth infections is not well understood. In addition, the common γc cytokines, IL-2, IL-4, IL-7, IL-9 and IL-15 play an important role in the maintenance of these CD4+ and CD8+ T cell subsets.

METHODS

To examine the major T cell subsets and their association with the common γc cytokines, the absolute numbers of CD4+ and CD8+ naïve, central memory, effector memory and effector cells and the plasma levels of IL-2, IL-4, IL-7, IL-9 and IL-15 were measured in Strongyloides stercoralis (Ss) infected (INF, n = 60), helminth-uninfected (UN, n = 58) and in post treatment INF individuals.

RESULTS

Ss infection is characterized by significantly increased absolute numbers of naïve and decreased absolute numbers of central and effector memory CD4+ T cells in comparison to UN individuals. No significant difference in the numbers of CD8+ T cell subsets was observed between the groups. The numbers of naïve cells and central memory CD4+ T cells were significantly reversed after anthelmintic treatment. Circulating levels of IL-2, IL-7 and IL-15 were significantly diminished, whereas the levels of IL-4 and IL-9 were significantly increased in INF compared to UN individuals. Following anthelminthic treatment, IL-2, IL-7 and IL-15 levels were significantly increased, while IL-4 and IL-9 levels were significantly decreased. Our data also showed a significant positive correlation between the levels of IL-7 and the numbers of central and effector memory CD4+ T cells.

CONCLUSION

Ss infection is characterized by alterations in the absolute numbers of CD4+ T cell subsets and altered levels of common γc cytokines IL-2, IL-4, IL-7, IL-9 and IL-15; alterations which are partially reversed after anthelmintic treatment.

Triggering immunological memory against the tapeworm Hymenolepis diminuta to protect against colitis

Infection with parasitic helminths can ameliorate the severity of concomitant inflammatory disease. To use the tapeworm, Hymenolepis diminuta, and to extend this concept by assessing whether triggering a memory response against the worm inhibits dinitrobenzene sulphonic acid (DNBS)-induced colitis in Balb/c mice. Initial studies revealed that oral infection with 1, 3 or 5 H. diminuta cysticercoids 8 days before intrarectal administration of DNBS (3 mg) resulted in less severe inflammation and that infected mice displayed an increased propensity for T helper-2 immunity. A 1 mg dose of a PBS-soluble extract of the worm (HdAg) delivered intraperitoneally concomitant with DNBS was anticolitic as determined by macroscopic and histological disease scores 72 hour post-DNBS. Mice infected 28 days previously had a memory response as determined by HdAg-evoked increases in interleukin (IL)-4 and IL-10 from in vitro stimulated splenocytes and serum anti-H. diminuta IgG. Moreover, mice infected with 5 H. diminuta 28 days previously were protected from DNBS-induced colitis by secondary infection or 100 μg HdAg (ip.) at the time of DNBS treatment. An additional approach to managing inflammatory disease could be infection with H. diminuta followed by eliciting antiworm recall responses.

Allergic Sensitization Underlies Hyperreactive Antigen-Specific CD4+ T Cell Responses in Coincident Filarial Infection

Among the various hypotheses put forward to explain the modulatory influence of helminth infection on allergic effector responses in humans, the IL-10-induced suppression of Th2-associated responses has been the leading candidate. To explore this helminth/allergy interaction more fully, parasite- and allergen-specific CD4(+) T cell responses in 12 subjects with filarial infections, and coincident allergic sensitization (filarial [Fil](+)allergy [A](+)) were compared with the responses to three appropriate control groups (Fil(-)A(-) [n = 13], Fil(-)A(+) [n = 12], Fil(+)A(-) [n = 11]). The most important findings revealed that Fil(+)A(+) had marked (p < 0.0001 for all cytokines) increases in parasite Ag-driven Th2 (IL-4, IL-5, IL-13), Th9 (IL-9), and the regulatory (IL-10) cytokines when compared with Fil(+)A(-) Moreover, using multiparameter flow cytometry, filarial parasite Ag induced a marked increase in not only the frequency of CD4(+) T cells producing IL-4, IL-5, IL-2, and TNF-α in Fil(+)A(+) when compared with Fil(+)A(-) patients, but also in the frequencies of polyfunctional Th2-like (CD4(+)IL-4(+)IL-5(+) and CD4(+)IL-2(+)IL-4(+)IL-5(+)TNF-α(+)) cells. The Th2-associated responses seen in the Fil(+)A(+) group were correlated with serum IgE levels (p < 0.01, r = 0.5165 for IL-4; p < 0.001, r = 0.5544 for IL-5; and p < 0.001, r = 0.4901 for IL-13) and levels of circulating eosinophils (p < 0.0116, r = 0.5656) and their degranulation/activation products (major basic protein [p < 0.001, r = 0.7353] and eosinophil-derived neurotoxin [p < 0.01, r = 0.7059]). CD4(+) responses to allergen were not different (to a large extent) among the groups. Taken together, our data suggest that allergic sensitization coincident with filarial infection drives parasite Ag-specific T cell hyperresponsiveness, which is characterized largely by an augmented Th2-dominated immune response.

Looking beyond the induction of Th2 responses to explain immunomodulation by helminths

Although helminth infections are characteristically associated with Th2-mediated responses that include the production of the prototypical cytokines IL-4, IL-5 and IL-13 by CD4(+) cells, the production of IgE, peripheral blood eosinophilia and mucus production in localized sites, these responses are largely attenuated when helminth infections become less acute. This modulation of the immune response that occurs with chronic helminth infection is often induced by molecules secreted by helminth parasites, by non-Th2 regulatory CD4(+) cells, and by nonclassical B cells, macrophages and dendritic cells. This review will focus on those parasite- and host-mediated mechanisms underlying the modulated T-cell response that occurs as the default in chronic helminth infections.

Latent tuberculosis infection--Revisiting and revising concepts

Host- and pathogen-specific factors interplay with the environment in a complex fashion to determine the outcome of infection with Mycobacterium tuberculosis (Mtb), resulting in one of three possible outcomes: cure, latency or active disease. Although much remains unknown about its pathophysiology, latent tuberculosis infection (LTBI) defined by immunologic evidence of Mtb infection is a continuum between self-cure and asymptomatic, yet active tuberculosis (TB) disease. Strain virulence, intensity of exposure to the index case, size of the bacterial inoculum, and host factors such as age and co-morbidities, each contribute to where one settles on the continuum. Currently, the diagnosis of LTBI is based on reactive tuberculin skin testing (TST) and/or a positive interferon-gamma release assay (IGRA). Neither diagnostic test reflects the activity of the infectious focus or the risk of progression to active TB. This is a critical shortcoming, as accurate and efficient detection of those with LTBI at higher risk of progression to TB disease would allow for provision of targeted preventive therapy to those most likely to benefit. Host biomarkers may prove of value in stratifying risk of development of TB. New guidelines are required for interpretation of discordance between TST and IGRA, which may be due in part to a lack of stability (that is reproducibility) of IGRA or TST results or to a delay in conversion of IGRA to positivity compared to TST. In this review, the authors elaborate on the definition, diagnosis, pathophysiology and natural history of LTBI, as well as promising methods for better stratifying risk of progression to TB. The review is centered on the human host and the clinical and epidemiologic features of LTBI that are relevant to the development of new and improved diagnostic tools.

Filarial infection modulates the immune response to Mycobacterium tuberculosis through expansion of CD4+ IL-4 memory T cells

Exaggerated CD4(+) T helper 2-specific cytokine producing memory T cell responses developing concomitantly with a T helper 1 response might have a detrimental role in immunity to infection caused by Mycobacterium tuberculosis. To assess the dynamics of Ag-specific memory T cell compartments in the context of filarial infection, we used multiparameter flow cytometry on PBMCs from 25 microfilaremic filarial-infected (Inf) and 14 filarial-uninfected (Uninf) subjects following stimulation with filarial Ag (BmA) or with the M. tuberculosis-specific Ag culture filtrate protein-10 (CFP-10). Our data demonstrated that the Inf group had a marked increase in BmA-specific CD4(+)IL-4(+) cells (median net frequency compared with baseline [Fo] = 0.09% versus 0.01%; p = 0.038) but also to CFP-10 (Fo = 0.16% versus 0.007%; p = 0.04) and staphylococcal enterotoxin B (Fo = 0.49% versus 0.26%; p = 0.04). The Inf subjects showed a BmA-specific expansion of CD4(+)CD45RO(+)IL-4(+) producing central memory (TCM, CD45RO(+)CCR7(+)CD27(+); Fo = 1.1% versus 0.5%; p = 0.04) as well as effector memory (TEM, CD45RO(+)CCR7(-)CD27(-); Fo = 1.5% versus 0.2%; p = 0.03) with a similar but nonsignificant response to CFP-10. In addition, there was expansion of CD4(+)IL-4(+)CD45RA(+)CCR7(+)CD27(+) (naive-like) in Inf individuals compared with Uninf subjects. Among Inf subjects with definitive latent tuberculosis, there were no differences in frequencies of IL-4-producing cells within any of the memory compartments compared with the Uninf group. Our data suggest that filarial infection induces Ag-specific, exaggerated IL-4 responses in distinct T cell memory compartments to M. tuberculosis-specific Ags, which are attenuated in subjects who are able to mount a delayed type hypersensitivity reaction to M. tuberculosis.

Leishmania braziliensis-reactive T cells are down-regulated in long-term cured cutaneous Leishmaniasis, but the renewal capacity of T effector memory compartments is preserved

Leishmania (Viannia) braziliensis control and tissue damage relate to the effector immune response, which in turn affects clinical outcome. Leishmania reactive CD4(+) and CD8(+) T cells are expanded in long-term healed cutaneous leishmaniasis (hCL) patients but their functional characteristics remain to be determined. This study investigates antigen-specific recall in long-term healed CL caused by L. braziliensis infection. Healed CL subjects were grouped according to the time elapsed since the end of therapy: less than two years and two to five years. Activation phenotype (CD69(+) or CD25(+)) and subpopulations of memory T cell phenotypes [central memory (Tcm): CD45RO(+) CCR7(+) or effector memory (Tem): CD45RO(+) CCR7(-)] were quantified in ex vivo blood mononuclear cells and after Leishmania antigens stimuli. A reduction in the percentage of activated Leishmania-responder CD4(+) and CD8(+) T cells in hCL was associated with the time elapsed since clinical cure. Percentage of CD69(+) in TCD4(+) and TCD8(+) cells were negatively correlated with IL-10 levels. Ex vivo analyses showed contracted Tem CD4(+) and Tem CD8(+) compartments from hCL with long time elapsed since clinical cure, although renewal of these compartments was observed following in vitro exposure to leishmanial stimuli. Our results show that healed L. braziliensis infected patients exhibit a recall response to Leishmania antigens with evident expansion of effector memory T cells. Regulated leishmanial-specific response seems to emerge only about two years after initial contact with the parasite antigens.

Th2 cell-intrinsic hypo-responsiveness determines susceptibility to helminth infection

The suppression of protective Type 2 immunity is a principal factor driving the chronicity of helminth infections, and has been attributed to a range of Th2 cell-extrinsic immune-regulators. However, the intrinsic fate of parasite-specific Th2 cells within a chronic immune down-regulatory environment, and the resultant impact such fate changes may have on host resistance is unknown. We used IL-4gfp reporter mice to demonstrate that during chronic helminth infection with the filarial nematode Litomosoides sigmodontis, CD4⁺ Th2 cells are conditioned towards an intrinsically hypo-responsive phenotype, characterised by a loss of functional ability to proliferate and produce the cytokines IL-4, IL-5 and IL-2. Th2 cell hypo-responsiveness was a key element determining susceptibility to L. sigmodontis infection, and could be reversed in vivo by blockade of PD-1 resulting in long-term recovery of Th2 cell functional quality and enhanced resistance. Contrasting with T cell dysfunction in Type 1 settings, the control of Th2 cell hypo-responsiveness by PD-1 was mediated through PD-L2, and not PD-L1. Thus, intrinsic changes in Th2 cell quality leading to a functionally hypo-responsive phenotype play a key role in determining susceptibility to filarial infection, and the therapeutic manipulation of Th2 cell-intrinsic quality provides a potential avenue for promoting resistance to helminths.

Helminth parasites mount chronic infections in over 1 billion people worldwide, of which filarial nematode infections account for 120 million. A major barrier to the development of protective Th2 immunity lies in the dominant down-regulatory immune responses invoked during infection. Although this immune suppression is linked with a range of Th2 cell-extrinsic immune regulators, the fate of CD4⁺ Th2 cells during chronic infection, and the role of Th2 cell-intrinsic regulation in defining protective immunity to infection is largely unknown. In this study, we use a murine model of filarial nematode infection to show that as infection progresses the Th2 effector cells responsible for killing helminths become functionally hypo-responsive, developing a phenotype similar to adaptive tolerance or exhaustion, and their ability to clear infection becomes impaired. We further demonstrate that we can therapeutically manipulate the intrinsic functional quality of hypo-responsive Th2 cells via the PD-1/PD-L2 co-inhibitory pathway to reawaken them and enhance resistance to infection. Thus, our data provide the first demonstration that Th2 cell-intrinsic hypo-responsiveness plays a key role in determining susceptibility to helminth infection.

Proportions of CD4+ memory T cells are altered in individuals chronically infected with Schistosoma haematobium

Characterisation of protective helminth acquired immunity in humans or experimental models has focused on effector responses with little work conducted on memory responses. Here we show for the first time, that human helminth infection is associated with altered proportions of the CD4+ memory T cells, with an associated alteration of T_H1 responses. The reduced CD4+ memory T cell proportions are associated with a significantly lower ratio of schistosome-specific IgE/IgG4 (marker for resistance to infection/re-infection) in uninfected older people. Helminth infection does not affect the CD8+ memory T cell pool. Furthermore, we show for the first time in a helminth infection that the CD4+ memory T cell proportions decline following curative anti-helminthic treatment despite increased CD4+ memory cell replication. Reduced accumulation of the CD4+ memory T cells in schistosome-infected people has implications for the development of natural or vaccine induced schistosome-specific protective immunity as well as for unrelated pathogens.

Regulation of global gene expression in human Loa loa infection is a function of chronicity

Background

Human filarial infection is characterized by downregulated parasite-antigen specific T cell responses but distinct differences exist between patients with longstanding infection (endemics) and those who acquired infection through temporary residency or visits to filarial-endemic regions (expatriates).

Methods and Findings

To characterize mechanisms underlying differences in T cells, analysis of global gene expression using human spotted microarrays was conducted on CD4⁺ and CD8⁺ T cells from microfilaremic Loa loa-infected endemic and expatriate patients. Assessment of unstimulated cells showed overexpression of genes linked to inflammation and caspase-associated cell death, particularly in endemics, and enrichment of the Th1/Th2 canonical pathway in endemic CD4⁺ cells. However, pathways within CD8⁺ unstimulated cells were most significantly enriched in both patient groups. Antigen (Ag)-driven gene expression was assessed to microfilarial Ag (MfAg) and to the nonparasite Ag streptolysin O (SLO). For MfAg-driven cells, the number of genes differing significantly from unstimulated cells was greater in endemics compared to expatriates (p<0.0001). Functional analysis showed a differential increase in genes associated with NFkB (both groups) and caspase activation (endemics). While the expatriate response to MfAg was primarily a CD4⁺ pro-inflammatory one, the endemic response included CD4⁺ and CD8⁺ cells and was linked to insulin signaling, histone complexes, and ubiquitination. Unlike the enrichment of canonical pathways in CD8⁺ unstimulated cells, both groups showed pathway enrichment in CD4⁺ cells to MfAg. Contrasting with the divergent responses to MfAg seen between endemics and expatriates, the CD4⁺ response to SLO was similar; however, CD8⁺ cells differed strongly in the nature and numbers (156 [endemics] vs 36 [expatriates]) of genes with differential expression.

Conclusions

These data suggest several important pathways are responsible for the different outcomes seen among filarial-infected patients with varying levels of chronicity and imply an important role for CD8⁺ cells in some of the global changes seen with lifelong exposure.

Infection with the filarial parasite Loa loa causes a parasite-specific downregulation of T cell responses. However, differences exist (clinical and immunologic) between patients born and living in filarial endemic regions (endemics) and those who become infected during travel or short-term residency (expatriates). T cell responses are more depressed in endemics while expatriates have more clinical “allergic-type” symptoms. In this study, we showed that these differences reflect transcriptional differences within the T cell compartment. Using microarrays, we examined global gene expression in both CD4⁺ and CD8⁺ T cells of microfilaremic endemic and expatriate patients and found differences not only ex vivo, but also to parasite and, for CD8⁺ cells, to nonparasite antigens. Functional analysis showed that endemic patients expressed genes linked to inflammatory disease and caspase associated cell death at homeostasis while expatriates tended to have a more activation-induced gene profile at homeostasis and a CD4⁺ inflammatory response to parasite antigen. Patient groups were similar in their CD4⁺ response to nonparasite antigen but strongly differed in their CD8⁺ responses, demonstrating the potential global ramifications of chronic, longstanding infection. Our study describes potential transcriptional mechanisms for the variability seen in patients with different levels of exposure to and chronicity of filarial infection.