CD4+ T‐cell activation is differentially modulated by bacteria‐primed dendritic cells,but is generally down‐regulated by n‐3 polyunsaturated fatty acids

Appropriate activation of CD4⁺ T cells is fundamental for efficient initiation and progression of acquired immune responses. Here, we showed that CD4⁺ T‐cell activation is dependent on changes in membrane n‐3 polyunsaturated fatty acids (PUFAs) and is dynamically regulated by the type of signals provided by dendritic cells (DCs). Upon interaction with DCs primed by different concentrations and species of gut bacteria, CD4⁺ T cells were activated according to the type of DC stimulus. The levels of CD80 were found to correlate to the levels of expression of CD28 and to the proliferation of CD4⁺ T cells, while the presence of CD40 and CD86 on DCs inversely affected inducible costimulator (ICOS) and cytotoxic T‐lymphocyte antigen‐4 (CTLA‐4) levels in CD4⁺ T cells. For all DC stimuli, cells high in n‐3 PUFAs showed reduced ability to respond to CD28 stimulation, to proliferate, and to express ICOS and CTLA‐4. Diminished T‐cell receptor (TCR) and CD28 signalling was found to be responsible for n‐3 PUFA effects. Thus, the dietary fatty acid composition influences the overall level of CD4⁺ T‐cell activation induced by DCs, while the priming effect of the DC stimuli modulates CD80, CD86 and CD40 levels, thereby affecting and shaping activation of acquired immunity by differential regulation of proliferation and costimulatory molecule expression in CD4⁺ T cells.     

Low breast milk levels of long‐chain n‐3 fatty acids in allergic women,despite frequent fish intake

Cite this as: S. Johansson, A. E. Wold and A‐S Sandberg, Clinical & Experimental Allergy, 2011 (41) 505–515.

Summary

Background Long‐chain n‐3 polyunsaturated fatty acids (PUFAs) have immune regulating and anti‐inflammatory effects. However, their role in allergic disease is unclear. Allergic diseases are immunologically heterogeneous, and we hypothesized that n‐3 fatty acid composition in serum and breast milk may vary according to clinical manifestations. Further, animal studies have shown reduction of serum‐PUFA levels during allergic inflammation. Objective To investigate fatty acid composition in breast milk and serum from women with different atopic disease manifestations. Secondly, to determine whether low PUFA levels reflected insufficient intakes. Methods Fatty acids were analysed in breast milk and serum of women with atopic eczema and respiratory allergy (n=16), only respiratory allergy (n=7), as well as healthy women (n=22). Dietary intake of foods expected to affect long‐chain n‐3 PUFA levels were estimated by food‐frequency questionnaire. The fatty acid pattern was related to diagnostic group and intake of relevant food items using a multivariate pattern recognition method (partial least squares projections to latent structures and discriminant analysis). Results Women with a combination of eczema and respiratory allergy had lower breast milk levels of several PUFAs (arachidonic acid, eicosapentaenoic acid, EPA, docosahexaenoic acid, DHA, and docosapentaenoic acid, DPA), and a lower ratio of long‐chain n‐3 PUFAs/n‐6 PUFAs. Their PUFA levels differed not only from that of healthy women, but also from that of women with only respiratory allergy. The latter had a fatty acid pattern similar to that of healthy women. Despite low EPA, DHA and DPA levels women with eczema and respiratory allergy consumed no less fish than did healthy women. Conclusion & Clinical Relevance Our data suggest that reduced levels of long‐chain n‐3 fatty acids in serum and breast milk characterize women with extensive allergic disease including eczema, and are not related to low fish intake. Consumption of PUFAs during the allergic process may explain these findings.     

Dendritic cells process synthetic long peptides better than whole protein,improving antigen presentation and T‐cell activation

The efficiency of antigen (Ag) processing by dendritic cells (DCs) is vital for the strength of the ensuing T‐cell responses. Previously, we and others have shown that in comparison to protein vaccines, vaccination with synthetic long peptides (SLPs) has shown more promising (pre‐)clinical results. Here, we studied the unknown mechanisms underlying the observed vaccine efficacy of SLPs. We report an in vitro processing analysis of SLPs for MHC class I and class II presentation by murine DCs and human monocyte‐derived DCs. Compared to protein, SLPs were rapidly and much more efficiently processed by DCs, resulting in an increased presentation to CD4⁺ and CD8⁺ T cells. The mechanism of access to MHC class I loading appeared to differ between the two forms of Ag. Whereas whole soluble protein Ag ended up largely in endolysosomes, SLPs were detected very rapidly outside the endolysosomes after internalization by DCs, followed by proteasome‐ and transporter associated with Ag processing‐dependent MHC class I presentation. Compared to the slower processing route taken by whole protein Ags, our results indicate that the efficient internalization of SLPs, accomplished by DCs but not by B or T cells and characterized by a different and faster intracellular routing, leads to enhanced CD8⁺ T‐cell activation.     

Scant activation of CD8 T cells by antigen loaded on heat shock protein

Heat shock proteins (HSP) not only function as chaperones for denatured proteins but also for antigenic peptides, thus inducing protective T cell responses. Here we show that vaccination with peptide-loaded HSP70 causes initial interferon-gamma production by murine CD8 T cells but no T cell expansion. These CD8 T cells lacked cytotoxic activity in vitro and in vivo, which was not due to apoptosis. Restimulation with peptide-pulsed dendritic cells both bypassed the proliferative block and suspended the non-protective state of CD8 T lymphocytes in an infection model with the bacterial pathogen, Listeria monocytogenes. Cotransfer of antigen-specific CD4 T cells circumvented the proliferative arrest of CD8 T cells. Our data suggest that HSP vaccines induce CD8 T cell unresponsiveness unless proficient help is provided. Assuming that this model reflects the antigenically experienced human condition where immunological space is restricted and any T cell response possibly leads to suppression of heterologous reactions, our findings bear implications for rational vaccination protocols including those for immunocompromised patients.     

Structure and duration of contact between dendritic cells and T cells are controlled by T cell activation state

The adaptive immune response is initiated when naive T cells interact with dendritic cells (DC). However, the physicodynamics as well as the molecules that constitute the contact plane (immunological synapse) between DC and T cells are not well understood. We show here that for the formation of stable conjugates, T cells need to be preactivated by DC in a CD80/86- and antigen dose-dependent manner. When activated, T cells induce cytoskeletal reorganization within DC via CD40-CD40L signaling. Polarization of the actin and fascin cytoskeleton in DC is associated with sustained DC-T cell contacts, strong T cell proliferation and a Th1 response. Organized contact planes with clearly separated patches containing TCR or CD11a are also formed. Thus, DC-T cell interactions take place in a sequential, interdependent fashion: first, DC "license" naive T cells to engage DC in an antigen dose- and CD80/86-dependent fashion. Then, these preactivated T cells induce cytoskeletal reorientation in DC, resulting in sustained DC-T cell contacts and subsequent T cell activation. These results demonstrate that T cells control the mode of interaction based on information gathered from DC.     

Differential recruitment and activation of natural killer cell sub‐populations by Mycobacterium bovis‐infected dendritic cells

Through complex interplay with APCs, subsets of NK cells play an important role in shaping adaptive immune responses. Bovine tuberculosis, caused by Mycobacterium bovis, is increasing in incidence and detailed knowledge of host–pathogen interactions in the natural host is essential to facilitate disease control. We investigated the interactions of NK‐cell sub‐populations and M. bovis‐infected DCs to determine early innate mechanisms in the response to infection. A sub‐population of NK cells (NKp46⁺CD2^?) selectively expressing lymphoid homing and inflammatory chemokine receptors were induced to migrate towards M. bovis‐infected DCs. This migration was associated with increased expression of chemokines CCL3, 4, 5, 20 and CXCL8 by M. bovis‐infected DCs. Activation of NKp46⁺CD2^? NK cells and secretion of IFN‐γ was observed, a response reliant on localised IL‐12 release and direct cellular interaction. In a reciprocal manner, NKp46⁺CD2^? cells induced an increase in the intensity of cell surface MHC class II expression on DCs. In contrast, NKp46⁺CD2⁺ NK cells were unable to secrete IFN‐γ and did not reciprocally affect DCs. This study provides novel evidence to demonstrate distinct effector responses between bovine NK‐cell subsets during mycobacterial infection.     

Embryonic stem cell‐derived haematopoietic progenitor cells down‐regulate the CD3 ξ chain on T cells,abrogating alloreactive T cells

Murine embryonic stem (ES) cell‐derived haematopoietic progenitor cells (HPCs) engraft and populate lymphoid organs. In vivo, HPCs engraft across MHC barriers protecting donor‐type allografts from rejection. However, the underlying phenomenon remains elusive. Here, we sought to determine the mechanism by which ES cell‐derived HPCs regulate alloreactive T cells. We used the 2C mouse, which expresses a transgenic T‐cell receptor against H2‐L^d to determine whether HPCs are deleted by cytotoxic T lymphocytes (CTLs). Previously, we reported that HPCs express MHC class I antigens poorly and do not express class II antigens. In vitro stimulated 2C CTLs failed to lyse H2‐L^d HPCs in a standard 4‐hr ⁵¹chromium release assay. Similarly, when the HPCs were tested in an ELISPOT assay measuring the release of interferon‐γ by CTLs, HPCs failed to induce CTL degranulation. In addition, mice that were injected with HPCs showed a marked decrease in T‐cell responses to alloantigen and CD3 stimulation, but showed a normal response to PMA/ionomycin, suggesting that HPCs impaired T‐cell signalling through the T‐cell receptor/CD3 complex. Here, we show that HPCs secrete arginase, an enzyme that scavenges l ‐arginine, leading to metabolites that down‐regulate CD3 ζ chain. Indeed an arginase inhibitor partially restored expression of the CD3 ζ chain, implicating arginase 1 in the down‐regulation of T cells. This previously unrecognized property of ES cell‐derived HPCs could positively enhance the engraftment of ES cell‐derived HPCs across MHC barriers by preventing rejection.     

Induction of immunosuppressive functions of dendritic cells in vivo by CD4+CD25+ regulatory T cells: role of B7-H3 expression and antigen presentation

Suppressive functions of CD4+CD25+ regulatory T cells (Treg) are mainly studied by their interaction with conventional T cells. However, there is evidence that Treg also interact with antigen-presenting cells (APC), leading to suppression of APC function in in vitro coculture systems. Studying the in vivo distribution of Treg after injection, we found that Treg are located in direct proximity to dendritic cells (DC) and affect their functional maturation status. After contact to Treg, DC up-regulate the inhibitory B7-H3 molecule and display reduced numbers of MHC-peptide complexes, leading to impaired T cell stimulatory function. When Treg-exposed DC were used to immunize animals against antigens, the DC failed to produce a robust immune response as compared to control DC. Thus, these data indicate that Treg are able to inhibit DC activation and produce an inhibitory phenotype of DC. Accordingly, Treg may recruit DC for the amplification of immunosuppression by restraining their maturation in vivo and inducing an immunosuppressive phenotype of DC.     

Maintenance of immune tolerance by Foxp3+ regulatory T cells requires CD69 expression

Although FoxP3⁺ regulatory T cells are key players in the maintenance of immune tolerance and autoimmunity, the lack of specific markers constitute an obstacle to their use for immunotherapy protocols. In this study, we have investigated the role of the C-type lectin receptor CD69 in the suppressor function of Tregs and maintenance of immune tolerance towards harmless inhaled antigens. We identified a novel FoxP3⁺CD69⁺ Treg subset capable to maintain immune tolerance and protect to developing inflammation. Although CD69⁺ and CD69⁻FoxP3⁺ Tregs exist in homeostasis, only CD69-expressing Tregs express high levels of CTLA-4, ICOS, CD38 and GITR suppression-associated markers, secrete high amounts of TGFβ and have potent suppressor activity. This activity is regulated by STAT5 and ERK signaling pathways and is impaired by antibody-mediated down-regulation of CD69 expression. Moreover, immunotherapy with FoxP3⁺CD69⁺ Tregs restores the homeostasis in Cd69^−/− mice, that fail to induce tolerance, and is also highly proficient in the prevention of inflammation. The identification of the FoxP3⁺CD69⁺ Treg subset paves the way toward the development of new therapeutic strategies to control immune homeostasis and autoimmunity.     

Treatment with Mesna and n‐3 polyunsaturated fatty acids ameliorates experimental ulcerative colitis in rats

Oxidative damage is a central feature of ulcerative colitis. Here, we tested whether the antioxidant Mesna, when administered alone or in combination with n‐3 polyunsaturated fatty acids (n‐3 PUFAs), affects the outcome of dextran sodium sulphate (DSS)‐induced ulcerative colitis in rats. After the induction of colitis, DSS‐treated rats were further treated orally (p.o), intraperitoneally (i.p) or intrarectally (i.r) for either 7 or 14 days with Mesna, n‐3 PUFAs or both. Rats were euthanized at the end of each treatment period. Clinical disease activity index was recorded throughout the experiment. At necropsy colorectal gross lesions were scored. Colitis was scored histologically, and the expression of myeloperoxidase (MPO), caspase‐3, inducible nitric oxide synthase (iNOS) and nuclear factor κB (NF‐κΒ) in colonic tissue was assessed by immunohistochemistry. Mesna alone was sufficient to significantly reduce colorectal tissue damage when administered orally or intraperitoneally. Orally coadministered n‐3 PUFAs enhanced this effect, resulting in the significant suppression of DSS colitis after 7 days, and a remarkable recovery of colorectal mucosa was evident after 14 days of treatment. The amelioration of colon pathology co‐existed with a significant decrease in MPO expression, overexpression of iNOS and reduction of nuclear NF‐κB p65 in inflammatory cells, and the suppression of apoptosis in colonic epithelial cells. The simultaneous administration of Mesna and n‐3 PUFAs is particularly effective in ameliorating DSS colitis in rats, by reducing oxidative stress, inflammation and apoptosis, probably through a mechanism that involves the inhibition of NF‐κB and overexpression of iNOS.     

Short‐chain fatty acids: Bacterial messengers modulating the immunometabolism of T cells

Short‐chain fatty acids (SCFAs) are mainly generated by bacterial fermentation of non‐digestible carbohydrates such as dietary fiber. In the last decade, new investigations have revealed that SCFAs have a very specific function and serve as active microbial metabolites, which are able to modulate the function of immune cells in the intestine and other tissues. Recent studies have highlighted the immunomodulatory potential of SCFAs in several autoimmune and inflammatory disorders such as multiple sclerosis, colitis, type 1 diabetes and rheumatoid arthritis. While the SCFA‐mediated activation of GPR41/GPR43 signalling pathways and their inhibitory activity on histone deacetylases have been extensively investigated, the impact of SCFAs on the T cell metabolism is poorly understood. SCFAs induce metabolic alterations in T cells by enhancing the activity of the mTOR complex and by regulating their glucose metabolism. Once taken up into T lymphocytes, SCFA‐derived acetyl groups contribute to the cellular acetyl‐CoA pool, which influences the histone acetylation and cytokine gene expression. This article reviews how SCFAs modulate the metabolic status of T cells, thereby impacting on epigenetic modifications and T cell function. We will also discuss how the recent findings from SCFA biology might be utilized for potential immune therapies of various autoimmune diseases.     

Priming of CD8+ T cell responses by pathogens typically depends on CD70-mediated interactions with dendritic cells

The CD27/CD70-interaction has been shown to provide a costimulatory and survival signal for T cells in vitro and in vivo. Recently, CD70 expression by DC was found to be important for the priming of CD8+ T cells. We show here that blocking CD70 interactions has a significant impact on priming of CD8+ T cell responses by vaccinia virus (VV), Listeria monocytogenes and vesicular stomatitis virus (VSV) in mice. However, the priming of specific CD8+ T cells upon infection with lymphocytic choriomeningitis virus (LCMV) was only marginally reduced by CD70-blockade. Blocking of CD70 prevented CD8+ T cell priming in DIETER mice, a model in which presentation of LCMV-derived epitopes can be induced selectively in dendritic cells (DC). In contrast, CD70-CD27 interactions were not important for the priming of VSV-specific CD4+ T cells or class switch of neutralizing antibodies. As we show that priming of CD8+ T cells by the pathogens used here is dependent on antigen presentation by DC and that infection results in up-regulation of CD70 on DC, we conclude that CD70 expression on DC plays an important role in the priming of CD8+ T cells by pathogens. Moreover, the lack of CD70 cannot be completely compensated for by other costimulatory molecules.     

CD161++CD8+ T cells,including the MAIT cell subset,are specifically activated by IL‐12+IL‐18 in a TCR‐independent manner

CD161⁺⁺CD8⁺ T cells represent a novel subset that is dominated in adult peripheral blood by mucosal‐associated invariant T (MAIT) cells, as defined by the expression of a variable‐α chain 7.2 (Vα7.2)‐Jα33 TCR, and IL‐18Rα. Stimulation with IL‐18+IL‐12 is known to induce IFN‐γ by both NK cells and, to a more limited extent, T cells. Here, we show the CD161⁺⁺ CD8⁺ T‐cell population is the primary T‐cell population triggered by this mechanism. Both CD161⁺⁺Vα7.2⁺ and CD161⁺⁺Vα7.2^? T‐cell subsets responded to IL‐12+IL‐18 stimulation, demonstrating this response was not restricted to the MAIT cells, but to the CD161⁺⁺ phenotype. Bacteria and TLR agonists also indirectly triggered IFN‐γ expression via IL‐12 and IL‐18. These data show that CD161⁺⁺ T cells are the predominant T‐cell population that responds directly to IL‐12+IL‐18 stimulation. Furthermore, our findings broaden the potential role of MAIT cells beyond bacterial responsiveness to potentially include viral infections and other inflammatory stimuli.     

Depletion of CD4+CD25+CD127lo regulatory T cells does not increase allergen‐driven T cell activation

Background It has been suggested that allergic diseases are caused by defective suppression of allergen‐specific Th2 cells by CD4⁺CD25⁺ regulatory T cells. However, such studies have been hampered by the difficulty in distinguishing regulatory T cells from CD25‐expressing activated T cells. Recently, it was shown that conventional T cells expressed high levels of CD127, whereas regulatory T cells were CD127^lo, allowing discrimination between these distinct T cell subpopulations. Objective The aim of this study was to study whether the putative regulatory subset defined as CD4⁺CD25⁺CD127^lo was involved in grass pollen‐reactive T cell responses. Methods Peripheral blood mononuclear cells (PBMCs) were obtained from allergic donors and non‐atopic controls out of season. Grass pollen‐induced cytokine production and proliferation were compared in cultures of undepleted cells and cells depleted of CD4⁺CD25⁺, CD4⁺CD25⁺CD127^hi or CD4⁺CD25⁺CD127^lo T cells. Results Undepleted cell cultures from allergic patients showed significantly increased proliferation and Th2 cytokine production compared with non‐atopic controls. Depletion of all CD25⁺ T cells did not increase cytokine production or proliferation, and more importantly, no increase in Th2 cytokine production or proliferation was observed in cell cultures depleted of CD4⁺CD25⁺CD127^lo cells (putative regulatory T cells) compared with undepleted PBMCs in both the allergic and the non‐atopic group. Conclusion Our study showed that T cells from grass pollen‐allergic patients and non‐atopic controls responded very differently to grass pollen extract, but this difference could not be explained by differences in regulatory T cell function. Further studies are needed to understand the importance of regulatory T cells in allergy.     

Superantigens initiate cognate CD4+ T cell/ B cell interactions leading to early activation and proliferation of B cells

Dimerization or even multimerization of various receptors is commonly required for signal transduction. We report here that clustering of major histocompatibility complex class II molecules in human B cells by biotinylated staphylococcal enterotoxin A (SEA) cross-linked with avidin induces an increase in the level of intracellular calcium. This response was abolished by prior treatment with protein tyrosine kinase (PTK) inhibitors, suggesting that SEA-triggered calcium mobilization in B cells is probably dependent on the activation of PTK. The implication of PTK in SEA-induced early B cell activation was then confirmed by demonstrating that cross-linked SEA induces a significant increase in the level of tyrosine phosphorylation in B cells. The requirement of biotinavidin cross-linking in SEA-induced calcium mobilization in B cells can be fulfilled by the addition CD4⁺ T cells, suggesting a role for CD4 molecules. Using the murine CD4^? T cell hybridoma 3DT, or its derivative 11B3 transfected with human CD4 that both express SEA-specific TCR, we confirmed the CD4 requirement for B cell calcium mobilization and that both specific TCR and CD4 molecules are required in early events of B cell activation induced by SEA. The role of CD4 in SEA-induced B cell proliferation was then investigated. SEA-stimulated B cells proliferated in the presence of CD4⁺ T cells, whereas no response was observed in the presence of CD8⁺ T cells. The addition of clone 11B3 CD4⁺ T cells failed to fulfill the requirement of CD4⁺ T cells in SEA-induced B cell proliferation, indicating the possible involvement of other CD4⁺ T cell surface molecules in this response. This issue is currently under investigation.     

A CD40/CD40L feedback loop drives the breakdown of CD8+ T‐cell tolerance following depletion of suppressive CD4+ T cells

Dendritic cells (DCs) are the key APCs not only for the priming of naïve T cells, but also for the induction and maintenance of peripheral T‐cell tolerance. We have recently shown that cognate interactions between Foxp3⁺ Tregs and steady‐state DCs are crucial to maintain the tolerogenic potential of DCs. Using DIETER mice, which allow the induction of antigen presentation selectively on DCs without altering their maturation status, we show here that breakdown of CD8⁺ T‐cell tolerance, which ensues after depletion of suppressive CD4⁺ T cells, is driven by a positive feedback loop in which autoreactive CD8⁺ T cells activate DCs via CD40. These data identify ligation of CD40 on DCs as a stimulus that promotes autoreactive T‐cell priming when regulatory T‐cell suppression fails and suggest that feedback from autoreactive T cells to DCs may contribute to the well‐documented involvement of CD40 in many autoimmune diseases.     

IL-21R expression on CD8+ T cells promotes CD8+ T cell activation in coxsackievirus B3 induced myocarditis

IL-21 is a multi-functional cytokine which can promote survival, proliferation and activation of T and B lymphocytes including CD8 T cells. Previous studies have shown that autoimmune CD8+ T cells are the primary pathogenic effector cell in coxsackievirus B3 (CVB3) induced myocarditis in C57Bl/6 mice. To evaluate the role of IL-21 in promoting CD8+ T cell mediated cardiac injury in myocarditis, C57Bl/6 and IL-21RKO mice were infected with CVB3. IL-21RKO mice developed significantly less myocarditis than C57Bl/6 animals although cardiac virus titers were equivalent between the mouse strains. Numbers of CD8+IFNγ+ cells were decreased in IL-21RKO mice but numbers of either CD4+IFNγ+ or CD4+IL-4+ cells were not significantly different from C57Bl/6 animals indicating a selective effect of IL-21 signaling on the CD8+ T cell response. To confirm that IL-21 signaling exclusively functions at the level of the CD8+ T cell in CVB3 induced myocarditis, purified CD8+ cells were isolated from either C57Bl/6 or IL-21RKO donors and adoptively transferred into CD8KO recipients prior to CVB3 infection. CD8KO recipients given either C57Bl/6 or IL-21RKO CD8+ cells showed equivalent reconstitution of the CD8+ cells in the spleen but the recipients given C57Bl/6 CD8+ cells showed significantly greater myocarditis than recipients of IL-21RKO CD8+ cells. These data demonstrate that IL-21 signaling directly in the CD8+ cell population is required for CVB3-induced myocarditis.     

IL‐10 promotes homeostatic proliferation of human CD8+ memory T cells and,when produced by CD1c+ DCs,shapes naive CD8+ T‐cell priming

IL‐10 is an anti‐inflammatory cytokine that inhibits maturation and cytokine production of dendritic cells (DCs). Although mature DCs have the unique capacity to prime CD8⁺ CTL, IL‐10 can promote CTL responses. To understand these paradoxic findings, we analyzed the role of IL‐10 produced by human APC subsets in T‐cell responses. IL‐10 production was restricted to CD1c⁺ DCs and CD14⁺ monocytes. Interestingly, it was differentially regulated, since R848 induced IL‐10 in DCs, but inhibited IL‐10 in monocytes. Autocrine IL‐10 had only a weak inhibitory effect on DC maturation, cytokine production, and CTL priming with high‐affinity peptides. Nevertheless, it completely blocked cross‐priming and priming with low‐affinity peptides of a self/tumor‐antigen. IL‐10 also inhibited CD1c⁺ DC‐induced CD4⁺ T‐cell priming and enhanced Foxp3 induction, but was insufficient to induce T‐cell IL‐10 production. CD1c⁺ DC‐derived IL‐10 had also no effect on DC‐induced secondary expansions of memory CTL. However, IL‐15‐driven, TCR‐independent proliferation of memory CTL was enhanced by IL‐10. We conclude that DC‐derived IL‐10 selects high‐affinity CTL upon priming. Moreover, IL‐10 preserves established CTL memory by enhancing IL‐15‐dependent homeostatic proliferation. These combined effects on CTL priming and memory maintenance provide a plausible mechanism how IL‐10 promotes CTL responses in humans.