IL‐7 is superior to IL‐2 for ex vivo expansion of tumour‐specific CD4+ T cells

It is well established that tumours hinder both natural and vaccine‐induced tumour‐specific CD4⁺ T‐cell responses. Adoptive T‐cell therapy has the potential to circumvent functional tolerance and enhance anti‐tumour protective responses. While protocols suitable for the expansion of cytotoxic CD8⁺ T cells are currently available, data on tumour‐specific CD4⁺ T cells remain scarce. We report here that CD4⁺ T cells sensitized to tumour‐associated Ag in vivo, proliferate in vitro in response to IL‐7 without the need for exogenous Ag stimulation and accumulate several folds while preserving a memory‐like phenotype. Both cell proliferation and survival accounts for the outgrowth of tumour‐sensitized T cells among other memory and naive lymphocytes following exposure to IL‐7. Also IL‐2, previously used to expand anti‐tumour CTL, promotes tumour‐specific CD4⁺ T‐cell accumulation. However, IL‐7 is superior to IL‐2 at preserving lymphocyte viability, in vitro and in vivo, maintaining those properties, that are required by helper CD4⁺ T cells to confer therapeutic efficacy upon transplantation in tumour‐bearing hosts. Together our data support a unique role for IL‐7 in retrieving memory‐like CD4⁺ T cells suitable for adoptive T‐cell therapy.     

Inhibition of Alloantigen‐Primed Memory CD4+ and CD8+ T Cells by Hematopoietic Chimerism in Mice

Donor‐reactive memory T cells present a special hurdle in transplantation. Although hematopoietic chimerism is effective for inducing donor‐specific tolerance, the effects on memory T cells are unclear. Here, we induced stable chimerism and tolerance in mice (Tolerance group, n = 6) by donor‐specific transfusion (DST) plus anti‐CD154 monoclonal antibody (mAb), avoiding the toxic myeloablative conditioning treatment to assist bone marrow transplantation (DST/aCD154&BMTx). We then transferred memory CD4⁺ or CD8⁺ T cells from donor antigen primed mice to the tolerance‐induced recipients 4 days after heart transplantation (Tol/CD4⁺ Tm group and Tol/CD8⁺ Tm group, n = 6, respectively), but neither of these memory T‐cell subsets had an effect on the permanent graft survival (median survival time > 100 days). The unaltered rate of memory T cells in spleen and anergy to donor antigen in vitro demonstrated that these memory T cells were well controlled. The chimerism‐promoting protocol DST/aCD154&BMTx produced an immune environment that included high levels of regulatory T cells (Tregs), microchimerism and TGF‐β, all of which may act in suppressing the donor‐reactive memory CD4⁺ or CD8⁺ T cells. These findings have potentially important implications for designing approaches to suppressing memory T cells for success of transplantation.     

CD69+ memory T lymphocytes of the bone marrow and spleen express the signature transcripts of tissue‐resident memory T lymphocytes

It is a matter of current debate whether the bone marrow is a hub for circulating memory T lymphocytes and/or the home of resident memory T lymphocytes. Here we demonstrate for CD69⁺ murine CD8⁺, and CD69⁺ murine and human CD4⁺ memory T lymphocytes of the bone marrow, making up between 30 and 60% of bone marrow memory T lymphocytes, that they express the gene expression signature of tissue‐resident memory T lymphocytes. This suggests that a substantial proportion of bone marrow memory T lymphocytes are resident. It adds to previous evidence that bone marrow memory T cells are resting in terms of mobility and proliferation, and maintain exclusive long‐term memory to distinct, systemic antigens.     

Analysis of the functional WT1‐specific T‐cell repertoire in healthy donors reveals a discrepancy between CD4+ and CD8+ memory formation

The Wilms’ tumour‐1 (WT1) protein is considered a prime target for cancer immunotherapy based on its presumptive immunogenicity and widespread expression across a variety of malignancies. However, little is known about the naturally occurring WT1‐specific T‐cell repertoire because self‐derived antigens typically elicit low frequency responses that challenge the sensitivity limits of current detection techniques. In this study, we used highly efficient cell enrichment procedures based on CD137, CD154, and pHLA class I tetramer staining to conduct a detailed analysis of WT1‐specific T cells from the peripheral blood. Remarkably, we detected WT1‐specific CD4⁺ and CD8⁺ T‐cell populations in the vast majority of healthy individuals. Memory responses specific for WT1 were commonly present in the CD4⁺ T‐cell compartment, whereas WT1‐specific CD8⁺ T cells almost universally displayed a naive phenotype. Moreover, memory CD4⁺ and naive CD8⁺ T cells with specificity for WT1 were found to coexist in some individuals. Collectively, these findings suggest a natural discrepancy between the CD4⁺ and CD8⁺ T‐cell lineages with respect to memory formation in response to a self‐derived antigen. Nonetheless, WT1‐specific T cells from both lineages were readily activated ex vivo and expanded in vitro, supporting the use of strategies designed to exploit this expansive reservoir of self‐reactive T cells for immunotherapeutic purposes.     

Suppression of murine tumour growth through CD8+ cytotoxic T lymphocytes via activated DEC‐205+ dendritic cells by sequential administration of α‐galactosylceramide in vivo

Cancer immunity is mediated through the effective priming and activation of tumour‐specific class I MHC molecule‐restricted CD8⁺ cytotoxic T lymphocytes (CTLs). DEC‐205⁺ dendritic cells (DCs) can cross‐present the epitope(s) of captured tumour antigens associated with class I MHC molecules alongside co‐stimulatory molecules to prime and activate tumour‐specific CD8⁺ CTLs. Immunosuppressive tolerogenic DCs with reduced co‐stimulatory molecules may be a cause of impaired CTL induction. Hepa1‐6‐1 cells were established from the mouse hepatoma cell line Hepa1‐6; these cells grow continuously after subcutaneous implantation into syngeneic C57BL/6 (B6) mice and do not prime CD8⁺ CTLs. In this study, we show that the growth of ongoing tumours was suppressed by activated CD8⁺ CTLs with tumour‐specific cytotoxicity through the administration of the glycolipid α‐galactosylceramide (α‐GalCer), which is a compound known to stimulate invariant natural killer T (iNKT) cells and selectively activate DEC‐205⁺ DCs. Moreover, we demonstrated that sequential repetitive intraperitoneal inoculation with α‐GalCer every 48 hr appeared to convert tolerogenic DEC‐205⁺ DCs into immunogenic DCs with a higher expression of co‐stimulatory molecules and a stronger cross‐presentation capacity, which primed CTL precursors and induced tumour‐specific CD8⁺ CTLs within the tumour environment without activating iNKT cells. These findings provide a new basis for cancer immunotherapy to convert tolerogenic DEC‐205⁺ DCs within tumours into immunogenic DCs through the sequential administration of an immuno‐potent lipid/glycolipid, and then activated immunogenic DCs with sufficient expression of co‐stimulatory molecules prime and activate tumour‐specific CD8⁺ CTLs within the tumour to control tumour growth.     

Atopic asthma: differential activation phenotypes among memory T helper cells

Background T cells have been implicated in the pathogenesis of atopic asthma. We have previously shown that memory T helper cells (CD4⁺CD45RO⁺) are preferentially activated relative to naïve T helper cells (CD4⁺CD45RA⁺) after bronchial allergen challenge. However, specific T helper subpopulations that are activated in atopy and/or asthma remain undefined. Objective To determine the T helper subpopulations and activation phenotypes relevant to acute and stable asthma that may be common with or distinct from atopy. Methods Two groups of atopic asthmatics (ten acute and nine stable asthmatics) and two non‐asthmatic groups (14 non‐asthmatic atopics and eight normal non‐atopic controls) were analysed. Ten acute asthmatics were assessed in the emergency room during an acute episode (FEV₁ 43.6% ± 18.4). Nine stable asthmatics were assessed during a symptom‐free period (FEV₁ 85% ± 6). Using multiple colour flow cytometry we analysed T cell subpopulations and the expression of IL‐2‐receptor (IL‐2R) and MHC‐class II antigens (MHC II) on naïve and memory T helper cells in the peripheral blood of asthmatic and non‐asthmatic groups. Results Atopic asthmatics (acute and stable) had an increased percentage of memory T helper cells expressing IL‐2R compared with normal non‐atopics (mean SD 16.1 ± 6%, 12.4 ± 2% and 7.7 ± 1.8%, P < 0.05) but not compared with non‐asthmatic atopics (10 ± 3.5%). Naïve T helper cells had low expression of IL‐2R and MHC II in all four groups. MHC II antigen expression was increased in memory T helper cells of asthmatics (acute and stable) compared with normal non‐atopics (13.9 ± 7.5, 10.6 ± 5 and 4.9 ± 2.5, P < 0.05) but not compared with non‐asthmatic atopics (7.92 4). A novel finding was that IL‐2R and the MHC II molecules were mainly expressed in non‐overlapping populations and coexpression was found predominantly on memory T helper cells. Asthmatics (acute and stable) had higher proportion of double positive memory T helper cells (IL‐2R⁺MHC II⁺) compared with both non‐asthmatic groups (P < 0.05). Conclusions We demonstrate a differential expression of IL‐2R⁺ and MCH II⁺ on CD45RO⁺ T helper cells that would suggest that there are three subsets of activated memory T helper cells in asthmatics. Two non‐overlapping IL‐2R⁺ or MHC II⁺ CD45RO⁺ T helper cells and a third subpopulation of activated cells that coexpress IL‐2R and MHC II (double positives). This latter subpopulation is significantly higher in asthmatics (acute or stable) compared with both non‐asthmatic groups, suggesting a specific T helper activation phenotype distinct to atopic asthmatics as compared with atopic non‐asthmatics.     

Bystander stimulation of activated CD4+ T cells of unrelated specificity following a booster vaccination with tetanus toxoid

Antigen‐specific CD4⁺ T cells are central to natural and vaccine‐induced immunity. An ongoing antigen‐specific T‐cell response can, however, influence surrounding T cells with unrelated antigen specificities. We previously observed this bystander effect in healthy human subjects following recall vaccination with tetanus toxoid (TT). Since this interplay could be important for maintenance of memory, we have moved to a mouse model for further analysis. We investigated whether boosting memory CD4⁺ T cells against TT in vivo would influence injected CD4⁺ TCR transgenic T cells (OT‐II) specific for an unrelated OVA peptide. If OT‐II cells were pre‐activated with OVA peptide in vitro, these cells showed a bystander proliferative response during the ongoing parallel TT‐specific response. Bystander proliferation was dependent on boosting of the TT‐specific memory response in the recipients, with no effect in naive mice. Bystander stimulation was also proportional to the strength of the TT‐specific memory T‐cell response. T cells activated in vitro displayed functional receptors for IL‐2 and IL‐7, suggesting these as potential mediators. This crosstalk between a stimulated CD4⁺ memory T‐cell response and CD4⁺ T cells activated by an unrelated antigen could be important in human subjects continually buffeted by environmental antigens.     

Dendritic cells activated by an anti‐inflammatory agent induce CD4+ T helper type 2 responses without impairing CD8+ memory and effector cytotoxic T‐lymphocyte responses

Prevalence of pro‐inflammatory diseases is rising in developed country populations. The increase in these diseases has fuelled the search for new, immune suppressive, anti‐inflammatory therapies, which do not impact, or minimally impact, CD4⁺ and/or CD8⁺ T‐cell‐mediated immunity. The goal of this study was to determine if antigen‐presenting cells (APCs) activated by the anti‐inflammatory oligosaccharide, lacto‐N‐fucopentaose III (LNFPIII), would have an impaired ability to drive CD4⁺ T helper (Th) or CD8⁺ memory and effector T‐cell responses. To investigate this we activated splenic dendritic cells (SDCs) with LNFPIII and examined their ability to drive antigen‐specific CD4⁺ Th, and CD8⁺ memory and cytotoxic T‐cell (CTL) responses compared with lipopolysaccharide (LPS) ‐stimulated SDCs. The LNFPIII‐activated SDCs had altered co‐stimulatory molecule expression compared with LPS‐stimulated SDCs, while the levels of SDC chemokines following activation by either compound were similar. LNFPIII‐activated SDCs produced significantly lower levels of interleukin‐12 but surprisingly higher levels of interleukin‐6 than LPS‐activated SDCs. Similar to previous studies using bone‐marrow‐derived DCs, LNFPIII‐activated SDCs induced strong Th2 responses in vivo and ex vivo. LNFPIII activation of APCs was independent of the Toll‐interleukin‐1 receptor adaptor myeloid differentiating factor 88. Importantly, LNFPIII‐matured DCs induced CD8⁺ memory and effector CTL responses similar to those driven by LPS‐matured DCs, including the frequency of interferon‐γ‐producing CD8⁺ T cells and induction of CTL effectors. Treatment of APCs by the anti‐inflammatory glycan LNFPIII did not impair their ability to drive CD8⁺ effector and memory cell‐mediated immunity.     

IL‐15 is critical for the maintenance and innate functions of self‐specific CD8+ T cells

IL‐15 is a pleiotropic cytokine involved in host defense as well as autoimmunity. IL‐15‐deficient mice show a decrease of memory phenotype (MP) CD8⁺ T cells, which develop naturally in naïve mice and whose origin is unclear. It has been shown that self‐specific CD8⁺ T cells developed in male H‐Y antigen‐specific TCR transgenic mice share many similarities with naturally occurring MP CD8⁺ T cells in normal mice. In this study, we found that H‐Y antigen‐specific CD8⁺ T cells in male but not female mice decreased when they were crossed with IL‐15‐deficient mice, mainly due to impaired peripheral maintenance. The self‐specific TCR transgenic CD8⁺ T cells developed in IL‐15‐deficient mice showed altered surface phenotypes and reduced effector functions ex vivo. Bystander activation of the self‐specific CD8⁺ T cells was induced in vivo during infection with Listeria monocytogenes, in which proliferation but not IFN‐γ production was IL‐15‐dependent. These results indicated important roles for IL‐15 in the maintenance and functions of self‐specific CD8⁺ T cells, which may be included in the naturally occurring MP CD8⁺ T‐cell population in naïve normal mice and participate in innate host defense responses.     

TLR2 engagement on memory CD8+ T cells improves their cytokine‐mediated proliferation and IFN‐γ secretion in the absence of Ag

Persistence of memory CD8⁺ T cells is known to be largely controlled by common gamma chain cytokines, such as IL‐2, IL‐7 and IL‐15. However, other molecules may be involved in this phenomenon. We show here that TLR2^?/? mice have a decreased frequency of memory phenotype CD8⁺ T cells when compared with WT mice. This prompted us to investigate the role of TLR2 in the homeostasis of memory CD8⁺ T cells. We describe here a new TLR2‐dependent mechanism which, in the absence of specific antigen, directly controls memory CD8⁺ T‐cell proliferation and IFN‐γ secretion. We demonstrate that TLR2 engagement on memory CD8⁺ T cells increases their proliferation and expansion induced by IL‐7 both in vitro and in vivo. We also show that TLR2 ligands act in synergy with IL‐2 to induce IFN‐γ secretion in vitro. Both conclusions are obtained with spontaneously arising memory phenotype and antigen‐specific memory CD8⁺ T cells. Altogether, our data support the idea that continuous TLR2 signaling in response to microbial stimuli or endogenous danger signals might directly contribute to the maintenance of the diversity memory CD8⁺ T cells in the organism.     

Evaluation of CD62L expression as a marker for vaccine-elicited memory cytotoxic T lymphocytes

The development of successful vaccination strategies for eliciting cytotoxic T lymphocytes (CTLs) will be facilitated by the definition of strategies for subdividing CTLs into functionally distinct subpopulations. We assessed whether surface expression of a number of cell‐surface proteins could be used to define functionally distinct subpopulations of memory CTLs in mice immunized with a recombinant vaccinia virus expressing human immunodeficiency virus (HIV)‐1 envelope (Env). We found changes in cell‐surface expression of CD11a, CD44, CD45RB, CD49d, CD54 and CD62L on Env‐specific CD8⁺ T cells that appeared to differentiate them from other CD8⁺ T cells within 1 week to 1 month following immunization. Further, we saw an up‐regulation of CD62L surface expression on Env‐specific CD8⁺ memory T cells several months after immunization. However, CD62L expression did not correlate with differences in the abilities of CTLs to proliferate or produce interferon gamma (IFN‐γ) and tumour necrosis factor alpha (TNF‐α) in vitro in response to Env peptide stimulation. Moreover, the expression of CD62L did not allow differentiation of CTLs into subpopulations with distinct expansion kinetics in vivo after adoptive transfer into naïve mice and subsequent boosting of these mice with a recombinant adenovirus expressing HIV‐1 Env. Therefore, the definition of memory CD8⁺ T‐cell subpopulations on the basis of CD62L expression in mice does not allow the delineation of functionally distinct CTL subpopulations.     

Is Secretion of Tumour‐specific Antigen Important for Cancer Eradication by CD4+ T Cells? – Implications for Cancer Immunotherapy by Adoptive T Cell Transfer

The potential for cancer immunotherapy by adoptive transfer of CD4⁺ T cells is gaining increased attention. Most cancer cells lack major histocompatibility complex (MHC) class II molecules and cannot present tumour‐specific antigens (TSA) directly to CD4⁺ T cells. We have reported that tumour‐specific CD4⁺ T cells collaborate with macrophages and dendritic cells. These professional antigen‐presenting cells endocytose and process TSA to display antigenic peptides on their MHC class II molecules for indirect cancer cell recognition by CD4⁺ T cells. We hypothesized that this critical step may depend on secretion of TSA by cancer cells. This was investigated in a mouse model for myeloma immunosurveillance mediated by CD4⁺ T cells. From this study, several conclusions could be drawn. First, TSA secretion facilitates cancer immunosurveillance. Second, TSA secretion results in stronger activation of naïve tumour‐specific CD4⁺ T cells in lymph nodes. Third, TSA concentration within the tumour extracellular matrix must reach a certain threshold to allow successful cancer immunosurveillance. Fourth, treatment by local injection of purified TSA enhances immunity against cancer cells that do not secrete TSA. Fifth, secretion of TSA by at least some cancer cells within a tumour favours antitumour immunity. Therefore, we propose that CD4⁺ T cells that recognize secreted TSA may be superior for immunotherapy by T cell transfer, because the local extracellular antigen concentration will be higher for secreted TSA. Thus, it is anticipated that secreted TSA will be more readily detected in vivo by transferred CD4⁺ T cells, resulting in more efficient tumour eradication.     

Splenic CD8α⁺ dendritic cells undergo rapid programming by cytosolic bacteria and inflammation to induce protective CD8⁺ T-cell memory

Memory CD8⁺ T lymphocytes are critical effector cells of the adaptive immune system mediating long‐lived pathogen‐specific protective immunity. Three signals – antigen, costimulation and inflammation – orchestrate optimal CD8⁺ T‐cell priming and differentiation into effector and memory cells and shape T‐cell functional fate and ability to protect against challenge infections. While among the conventional spleen DCs (cDCs), the CD8α⁺ but not the CD8α^? cDCs most efficiently mediate CD8⁺ T‐cell priming, it is unclear which subset, irrespective of their capacity to process MHC class I‐associated antigens, is most efficient at inducing naïve CD8⁺ T‐cell differentiation into pathogen‐specific protective memory cells in vivo. Moreover, the origin of the required signals is still unclear. Using mice infected with the intracellular bacterium Listeria monocytogenes, we show that splenic CD8α⁺ cDCs become endowed with all functional features to optimally prime protective memory CD8⁺ T cells in vivo within only a few hours post‐immunization. Such programming requires both cytosolic signals resulting from bacterial invasion of the host cells and extracellular inflammatory mediators. Thus, these data designate these cells as the best candidates to facilitate the development of cell‐based vaccine therapy.     

On integrity in immunity during ontogeny or how thymic regulatory T cells work

The Standard model of T cell recognition asserts that T cell receptor (TCR) specificities are positively and negatively selected during ontogeny in the thymus and that peripheral T cell repertoire has mild self‐major histocompatibility complex (MHC) reactivity, known as MHC restriction of foreign antigen. Thus, the TCR must bind both a restrictive molecule (MHC allele) and a peptide reclining in its groove (pMHC ligand) in order to transmit signal into a T cell. The Standard and Cohn's Tritope models suggest contradictory roles for complementarity‐determining regions (CDRs) of the TCRs. Here, I discuss both concepts and propose a different solution to ontogenetic mechanism for TCR‐MHC–conserved interaction. I suggest that double (CD4⁺CD8⁺)‐positive (DP) developing thymocytes compete with their αβTCRs for binding to self‐pMHC on cortical thymic epithelial cells (cTECs) that present a selected set of tissue‐restricted antigens. The competition between DPs involves TCR editing and secondary rearrangements, similar to germinal‐centre B cell somatic hypermutation. These processes would generate cells with higher TCR affinity for self‐pMHC, facilitating sufficiently long binding to cTECs to become thymic T regulatory cells (tTregs). Furthermore, CD4⁺ Foxp3⁺ tTregs can be generated by mTECs via Aire‐dependent and Aire‐independent pathways, and additionally on thymic bone marrow–derived APCs including thymic Aire‐expressing B cells. Thymic Tregs differ from the induced peripheral Tregs, which comprise the negative feedback loop to restrain immune responses. The implication of thymocytes’ competition for the highest binding to self‐pMHC is the co‐evolution of species‐specific αβTCR V regions with MHC alleles.     

Interleukin‐4‐induced loss of CD8 expression and cytolytic function in effector CD8 T cells persists long term in vivo

Activation of naive CD8⁺ T cells in the presence of interleukin‐4 modulates their CD8 co‐receptor expression and functional differentiation, resulting in the generation of CD8^low cells that produce type 2 cytokines and display poor cytolytic and anti‐tumour activity. Although this CD8^low phenotype becomes stable after about a week and can persist with further stimulation in vitro, it is not known whether it can be maintained long term in vivo. Here we report that CD8^low cells derived from oval‐bumin_257–264‐specific T‐cell receptor‐transgenic CD8⁺ T cells activated in the presence of interleukin‐4 could be detected in the spleen for at least 4 months after adoptive transfer into normal mice. A significant proportion of the long‐term surviving cells retained their CD8^low phenotype in vivo and after clonal re‐activation in vitro. Although long‐term surviving CD8^low cells lacked detectable cytolytic activity or perforin expression, they showed some anti‐tumour function in vivo. The persistence of functional cells with a CD8^low phenotype in vivo raises the possibility that such cells can contribute to effector or regulatory responses to tumours or pathogens.     

Cytomegalovirus drives Vδ2neg γδ T cell inflation in many healthy virus carriers with increasing age

Cytomegalovirus (CMV) usually causes lifelong asymptomatic infection, but over time can distort immune profiles. Recent reports describe selective expansion of Vδ2^neg γδ T cells in healthy and immunocompromised CMV carriers. Having shown previously that virus‐specific CD8⁺ and CD4⁺ T cell responses are increased significantly in elderly CMV carriers, probably driven by chronic stimulation, we hypothesized that Vδ2^neg γδ T cells may also be expanded with age. Our results show that Vδ2^neg γδ T cells are increased significantly in CMV‐seropositive healthy individuals compared to CMV‐seronegative controls in all age groups. The differences were most significant in older age groups (P < 0·0001). Furthermore, while Vδ2^neg γδ T‐ cells comprise both naive and memory cells in CMV‐seronegative donors, highly differentiated effector memory cells are the dominant phenotype in CMV carriers, with naive cells reduced significantly in numbers in CMV‐seropositive elderly. Although phenotypically resembling conventional CMV‐specific T cells, Vδ2^neg γδ T cells do not correlate with changes in magnitude of CMV‐specific CD4⁺ or CD8⁺ T cell frequencies within those individuals, and do not possess ex‐vivo immediate effector function as shown by CMV‐specific CD4⁺ and CD8⁺ T cells. However, after short‐term culture, Vδ2^neg γδ T cells demonstrate effector T cell functions, suggesting additional requirements for activation. In summary, Vδ2^neg γδ T cells are expanded in many older CMV carriers, demonstrating a further level of lymphocyte subset skewing by CMV in healthy individuals. As others have reported shared reactivity of Vδ2^neg γδ T cells towards tumour cells, the composition of γδ T cell subsets may also have implications for risk of developing cancer in elderly people.     

Effector and memory CD8+ T cells as seen in immunity to malaria

Summary: Transgenic (Tg) mice carrying a T‐cell receptor (TCR) specific for a CD8⁺ T‐cell epitope expressed in pre‐erythrocytic stages of Plasmodium yoelii has proven to be a valuable tool to advance our understanding of this anti‐parasite T‐cell response, as it occurs in vivo. The visualization of CD8⁺ T cells in vivo and ex vivo greatly facilitated research aimed at characterizing basic features of this T‐cell response such as the kinetics of differentiation and proliferation and the in vivo antigen presentation. Importantly, this research unveiled the existence of early self‐regulatory mechanisms controlling the magnitude of the CD8⁺ T‐cell response and also identified CD4⁺ T cells as critical elements in the development of memory populations. This review discusses our recent research using Tg mice and highlights our progress in understanding the CD8⁺ T‐cell‐mediated immunity against malaria liver stages.     

Positive selection of self‐antigen‐specific CD8+ T cells by hematopoietic cells

In contrast to thymic epithelial cells, which induce the positive selection of conventional CD8⁺ T cells, hematopoietic cells (HCs) select innate CD8⁺ T cells whose Ag specificity is not fully understood. Here we show that CD8⁺ T cells expressing an H‐Y Ag‐specific Tg TCR were able to develop in mice in which only HCs expressed MHC class I, when HCs also expressed the H‐Y Ag. These HC‐selected self‐specific CD8⁺ T cells resemble innate CD8⁺ T cells in WT mice in terms of the expression of memory markers and effector functions, but are phenotypically distinct from the thymus‐independent CD8⁺ T‐cell population. The peripheral maintenance of H‐Y‐specific CD8⁺ T cells required presentation of the self‐Ag and IL‐15 on HCs. HC‐selected CD8⁺ T cells in mice lacking the Tg TCR also showed these features. Furthermore, by using MHC class I tetramers with a male Ag peptide, we found that self‐Ag‐specific CD8⁺ T cells in TCR non‐Tg mice could develop via HC‐induced positive selection, supporting results obtained from H‐Y TCR Tg mice. These findings indicate the presence of self‐specific CD8⁺ T cells that are positively selected by HCs in the peripheral T‐cell repertoire.     

Naive Tumour‐Specific CD4+ T Cells were Efficiently Primed in Acute Lymphoblastic Leukaemia

The recognition and neutralization of tumour cells is one of the big challenges in immunity. The immune system has to recognize syngeneic tumour cells and has to be primed and respond in an adequate manner. Priming of a leukaemia‐specific immune response is a crucial step in tumour immunology that can mislead to tumour tolerance either by T cell ignorance, deletion or Treg induction. To resemble the situation of acute lymphoblastic leukaemia (ALL) in patients, we used the murine BALB/c model with syngeneic BM185 tumour cells. We established a tumour cell line that expresses the neo‐antigen ovalbumin (BM185‐OVA/GFP) to allow the application of T cell receptor transgenic, antigen‐specific CD4⁺ T cells. Here, we demonstrate that effective anti‐ALL immunity can be established by in vivo priming of CD4⁺ T cells that is sufficient to differentiate into effector cells. Yet they failed to control tumour alone, but initiated a Th1 response. An efficient tumour clearance was dependent on both antigen‐specific CD4⁺ T cells and CD8⁺ effector T cells from the endogenous repertoire. The tolerogeneic milieu was characterized by increased Tregs numbers and elevated IL‐10 level. Tregs hamper effective antitumour immune response, but their depletion did not result in reduced tumour growth. In contrast, neutralization of IL‐10 improved median mouse survival. Future therapies should focus on establishing a strong CD4+ T cells response, either by adjuvant or by adoptive transfer.