Characterization of the human CD4+ T‐cell repertoire specific for major histocompatibility class I‐restricted antigens

While CD4⁺ T lymphocytes usually recognize antigens in the context of major histocompatibility (MHC) class II alleles, occurrence of MHC class‐I restricted CD4⁺ T cells has been reported sporadically. Taking advantage of a highly sensitive MHC tetramer‐based enrichment approach allowing detection and isolation of scarce Ag‐specific T cells, we performed a systematic comparative analysis of HLA‐A*0201‐restricted CD4⁺ and CD8⁺ T‐cell lines directed against several immunodominant viral or tumoral antigens. CD4⁺ T cells directed against every peptide‐MHC class I complexes tested were detected in all donors. These cells yielded strong cytotoxic and T helper 1 cytokine responses when incubated with HLA‐A2⁺ target cells carrying the relevant epitopes. HLA‐A2‐restricted CD4⁺ T cells were seldom expanded in immune HLA‐A2⁺ donors, suggesting that they are not usually engaged in in vivo immune responses against the corresponding peptide‐MHC class I complexes. However, these T cells expressed TCR of very high affinity and were expanded following ex vivo stimulation by relevant tumor cells. Therefore, we describe a versatile and efficient strategy for generation of MHC class‐I restricted T helper cells and high affinity TCR that could be used for adoptive T‐cell transfer‐ or TCR gene transfer‐based immunotherapies.     

Competition between T cells maintains clonal dominance during memory inflation induced by MCMV

Both human cytomegalovirus (HCMV) and murine cytomegalovirus (MCMV) establish persistent infections that induce the accumulation of virus‐specific T cells over time in a process called memory inflation. It has been proposed that T cells expressing T‐cell receptors (TCRs) with high affinity for HCMV‐derived peptides are preferentially selected after acute HCMV infection. To test this in the murine model, small numbers of OT‐I transgenic T cells, which express a TCR with high affinity for the SIINFEKL peptide, were transferred into congenic mice and recipients were challenged with recombinant MCMV expressing SIINFEKL. OT‐I T cells were selectively enriched during the first 3 weeks of infection. Similarly, in the absence of OT‐I T cells, the functional avidity of SIINFEKL‐specific T cells increased from early to late times postinfection. However, even when exceedingly small numbers of OT‐I T cells were transferred, their inflation limited the inflation of host‐derived T cells specific for SIINFEKL. Importantly, subtle minor histocompatibility differences led to late rejection of the transferred OT‐I T cells in some mice, which allowed host‐derived T cells to inflate substantially. Thus, T cells with a high functional avidity are selected shortly after MCMV infection and continuously sustain their clonal dominance in a competitive manner.     

Adaptive reconfiguration of the human NK‐cell compartment in response to cytomegalovirus: A different perspective of the host‐pathogen interaction

As discussed in this review, human cytomegalovirus (HCMV) infection in healthy individuals is associated with a variable and persistent increase of NK cells expressing the CD94/NKG2C activating receptor. The expansion of NKG2C⁺ NK cells reported in other infectious diseases is systematically associated with HCMV co‐infection. The functionally mature NKG2C^bright NK‐cell subset expanding in HCMV⁺ individuals displays inhibitory Ig‐like receptors (KIR and LILRB1) specific for self HLA class I, and low levels of NKp46 and NKp30 activating receptors. Such reconfiguration of the NK‐cell compartment appears particularly marked in immunocompromised patients and in children with symptomatic congenital infection, thus suggesting that its magnitude may be inversely related with the efficiency of the T‐cell‐mediated response. This effect of HCMV infection is reminiscent of the pattern of response of murine Ly49H⁺ NK cells against murine CMV (MCMV), and it has been hypothesized that a cognate interaction of the CD94/NKG2C receptor with HCMV‐infected cells may drive the expansion of the corresponding NK‐cell subset. Yet, the precise role of NKG2C⁺ cells in the control of HCMV infection, the molecular mechanisms underlying the NK‐cell compartment redistribution, as well as its putative influence in the response to other pathogens and tumors remain open issues.     

IL‐15 modulates the balance between Bcl‐2 and Bim via a Jak3/1‐PI3K‐Akt‐ERK pathway to promote CD8αα+ intestinal intraepithelial lymphocyte survival

IL‐15 is an essential survival factor for CD8αα⁺ intestinal intraepithelial lymphocytes (iIELs) in vitro and in vivo. However, the IL‐15‐induced survival signals in primary CD8αα⁺ iIELs remains elusive. Although Bcl‐2 level in CD8αα⁺ iIELs positively correlates with IL‐15Rα expression in the intestinal epithelial cells, overexpression of Bcl‐2 only moderately restores CD8αα⁺ γδ iIELs in Il15^?/? mice. Here, we found that IL‐15 promptly activated a Jak3‐Jak1‐PI3K‐Akt pathway that led to the upregulation of Bcl‐2 and Mcl‐1. This pathway also induced a delayed but sustained ERK1/2 activation, which not only was necessary for the maintenance of Bcl‐2 but also resulted in the phosphorylation of extra‐long Bim at Ser⁶⁵. The latter event facilitated the dissociation of Bim from Bcl‐2 without affecting Bim abundance in IL‐15‐treated CD8αα⁺ iIELs. Using an adoptive cell transfer approach, we found that either overexpression of Bcl‐2 or removal of Bim from CD8αα⁺ iIELs promoted their survival in Il15ra^?/? mice. Taken together, IL‐15 promotes CD8αα⁺ iIEL survival by both increasing Bcl‐2 levels and dissociating Bim from Bcl‐2 through activation of a Jak3‐Jak1‐PI3K‐Akt‐ERK1/2 pathway, which differs from a previously reported IL‐15‐induced survival signal.     

Communication between human mast cells and CD4+ T cells through antigen‐dependent interactions

Mast cells (MCs) are immune cells residing in tissues where pathogens are first encountered. It has been indicated that MCs might also be involved in setting the outcome of T‐cell responses. However, little is known about the capacity of human MCs to express MHC class II and/or to capture and present antigens to CD4⁺ T cells. To study the T‐cell stimulatory potential of human MCs, CD34⁺ stem cell derived MCs were generated. These cells expressed HLA‐DR when stimulated with IFN‐γ, and, importantly, presented peptide and protein for activation of antigen‐specific CD4⁺ T cells. The interplay between MC and T cell led to increased HLA‐DR expression on MCs. MCs were present in close proximity to T cells in tonsil and expressed HLA‐DR and CD80, indicating their ability to present antigens to CD4⁺ T cells in T‐cell areas of human LNs. Our data show that MCs can present native antigens to human CD4⁺ T cells and that HLA‐DR expressing MCs are present in tonsil tissue, indicating that human MCs can directly activate T cells and provide a rationale to study the potential of MCs to prime and/or skew human T‐cell responses.     

The choice of adjuvant determines the cytokine profile of T cells in proteoglycan‐induced arthritis but does not influence disease severity

Rheumatoid arthritis (RA) is a debilitating autoimmune disease characterized by chronic inflammation of the synovial joints. Collagen‐induced arthritis (CIA) and proteoglycan‐induced arthritis (PGIA) are mouse models of inflammatory arthritis; CIA is a T helper type 17 (Th17) ‐dependent disease that is induced with antigen in complete Freund's adjuvant, whereas PGIA is Th1‐mediated and is induced using antigen in dimethyldioctadecyl‐ammonium bromide (DDA) as an adjuvant. To investigate whether the type of adjuvant determines the cytokine profile of the pathogenic T cells, we have compared the effect of CFA and DDA on T‐cell responses in a single arthritis model. No differences in incidence or disease severity between aggrecan‐T‐cell receptor transgenic mice immunized with aggrecan in either CFA or DDA were observed. Immunization with CFA resulted in a higher proportion of Th17 cells, whereas DDA induced more Th1 cells. However, the levels of interleukin‐17 (IL‐17) produced by T cells isolated from CFA‐immunized mice after antigen‐specific stimulation were not significantly different from those found in DDA‐immunized mice, indicating that the increased proportion of Th17 cells did not result in significantly higher ex vivo IL‐17 levels. Hence, the choice of adjuvant can affect the overall proportions of Th1 and Th17 cells, without necessarily affecting the level of cytokine production or disease incidence and severity.     

Differential requirements of CD4+ T‐cell signals for effector cytotoxic T‐lymphocyte (CTL) priming and functional memory CTL development at higher CD8+ T‐cell precursor frequency

Increased CD8⁺ T‐cell precursor frequency (PF) precludes the requirement of CD4⁺ helper T (Th) cells for primary CD8⁺ cytotoxic T‐lymphocyte (CTL) responses. However, the key questions of whether unhelped CTLs generated at higher PF are functional effectors, and whether unhelped CTLs can differentiate into functional memory cells at higher PF are unclear. In this study, ovalbumin (OVA) ‐pulsed dendritic cells (DC_OVA) derived from C57BL/6, CD40 knockout (CD40^?/?) or CD40 ligand knockout (CD40L^?/?) mice were used to immunize C57BL/6, Ia^b?/?, CD40^?/? or CD40L^?/? mice, whose PF was previously increased with transfer of 1 × 10⁶ CD8⁺ T cells derived from OVA‐specific T‐cell receptor (TCR) transgenic OTI, OTI(CD40^?/?) or OTI(CD40L^?/?) mice. All the immunized mice were then assessed for effector and memory CTL responses. Following DC immunization, relatively comparable CTL priming occurred without CD4⁺ T‐cell help and Th‐provided CD40/CD40L signalling. In addition, the unhelped CTLs were functional effectors capable of inducing therapeutic immunity against established OVA‐expressing tumours. In contrast, the functional memory development of CTLs was severely impaired in the absence of CD4⁺ T‐cell help and CD40/CD40L signalling. Finally, unhelped memory CTLs failed to protect mice against lethal tumour challenge. Taken together, these results demonstrate that CD4⁺ T‐cell help at higher PF, is not required for effector CTL priming, but is required for functional memory CTL development against cancer. Our data may impact the development of novel preventive and therapeutic approaches in cancer patients with compromised CD4⁺ T‐cell functions.     

Sialoadhesin – a macrophage‐restricted marker of immunoregulation and inflammation

Sialoadhesin (Sn, also known as Siglec‐1 and CD169) is a macrophage‐restricted cell surface receptor that is conserved across mammals. Sn is a member of the sialic acid‐binding IgG‐like lectin (Siglec) family of proteins characterized by affinity to specifically sialylated ligands, and under normal conditions is expressed on subsets of macrophages in secondary lymphoid tissues, such as lymph node and spleen. However, Sn‐positive macrophages can also be found in a variety of pathological conditions, including (autoimmune) inflammatory infiltrates and tumours. Sn has been shown to contribute to sialylated pathogen uptake, antigen presentation and lymphocyte proliferation, and to influence both immunity and tolerance. This review presents Sn as a macrophage‐specific marker of inflammation and immunoregulation with the potential to becoming an important biomarker for immunologically active macrophages and a target for therapy.