Non‐neutralizing antibodies protect from chronic LCMV infection independently of activating FcγR or complement

Chronic viral infections lead to CD8⁺ T cell exhaustion, characterized by impaired cytokine secretion. The presence of the immune‐regulatory cytokine IL‐10 promotes chronic lymphocytic choriomeningitis virus (LCMV) Clone 13 infection in mice, whereas the absence of IL‐10/IL‐10R signaling early during infection results in viral clearance and higher percentages and numbers of antiviral, cytokine‐producing T cells. However, it is currently unclear which cell populations and effector molecules are crucial to protect against chronic infection. In this study, we demonstrate that antiviral, LCMV‐binding, non‐neutralizing antibodies are needed, in addition to CD4⁺ and CD8⁺ T cells, to clear a high‐dose LCMV infection in mice, in the absence of IL‐10. The interaction between CD4⁺ T cells and B cells in B‐cell follicles via CD40/CD40L, in addition to class switch and/or somatic hypermutation, is crucial for viral control in the absence of IL‐10. Interestingly, transfer of LCMV‐binding non‐neutralizing antibodies protected recipients from chronic infection. In addition, viral clearance in the absence of IL‐10R signaling was independent of activating Fcγ receptors and complement. These data highlight that non‐neutralizing antibodies effectively contribute to the control of LCMV infection when present prior to infection, suggesting that the induction of neutralizing antibodies is not implicitly necessary for the generation of successful vaccines.     

CD8 T cell immunodominance shifts during the early stages of acute LCMV infection independently from functional avidity maturation

Virus-specific T cell responses are often directed to a small subset of possible epitopes and their relative magnitude defines their hierarchy. We determined the size and functional avidity of 4 representative peptide-specific CD8⁺ T cell populations in C57BL/6 mice at different time points after lymphocytic choriomeningitis virus (LCMV) infection. We found that the frequency of different peptide-specific T cell populations in the spleen changed independently over the first 8 days after infection. These changes were not associated with a larger or more rapid increase in functional avidity and yet still resulted in a shift in the final immunodominance hierarchy. Thus, the immunodominance observed at the peak of an antiviral T cell response is not necessarily determined by the initial size or rate of functional avidity maturation of peptide-specific T cell populations.     

Blood and beyond: Properties of circulating and tissue‐resident human virus‐specific αβ CD8+ T cells

CD8⁺ αβ T‐cell responses form an essential line of defence against viral infections. An important part of the mechanisms that control the generation and maintenance of these responses have been elucidated in experimental mouse models. In recent years it has become clear that CD8⁺ T‐cell responses in humans not only show similarities, but also display differences to those occurring in mice. Furthermore, while several viral infections occur primarily in specialised organ systems, for obvious reasons, most human CD8⁺ T‐cell investigations were performed on cells deriving from the circulation. Indeed, several lines of evidence now point to essential functional differences between virus‐specific CD8⁺ memory T cells found in the circulation and those providing protection in organ systems, such as the lungs. In this review, we will focus on summarising recent insights into human CD8⁺ T‐cell differentiation in response to several viruses and emphasise that for a complete understanding of anti‐viral immunity, it is pivotal to scrutinise such responses in both blood and tissue.     

Residual LCMV antigen in transiently CD4+ T cell‐depleted mice induces high levels of virus‐specific antibodies but only limited B‐cell memory

Infection of C57BL/6 mice with lymphocytic choriomeningitis virus (LCMV) strain Armstrong (Arm) induces an acute infection with rapid virus clearance by CD8⁺ T cells independently of CD4⁺ T cell help. Residual viral antigen may, however, persist for a prolonged time. Here, we demonstrate that mice that had been transiently depleted of CD4⁺ T cells during acute LCMV Arm infection generated high levels of virus‐specific IgG antibodies (Ab) after viral clearance. Robust induction of LCMV‐specific IgG after transient CD4⁺ T cell depletion was dependent on Fcγ receptors but not on the complement receptors CD21/CD35. In contrast to the potent production of LCMV‐specific IgG, the generation of LCMV‐specific isotype‐switched memory B cells after transient CD4⁺ T cell depletion was considerably reduced. Moreover, mice depleted of CD4⁺ T cells during acute infection were strongly impaired in generating a secondary LCMV‐specific B cell response upon LCMV rechallenge. In conclusion, our data indicate that LCMV antigen depots after viral clearance were capable of inducing high levels of virus‐specific IgG. They failed, however, to induce robust virus‐specific B cell memory revealing a previously unappreciated dichotomy of specific Ab production and memory cell formation after priming with residual antigen.     

Low‐affinity FcγR interactions can decide the fate of novel human IgG‐sensitised red blood cells and platelets

G1Δnab is a mutant human IgG1 constant region with a lower ability to interact with FcγR than the natural IgG constant regions. Radiolabelled RBCs and platelets sensitised with specific G1Δnab Abs were cleared more slowly from human circulation than IgG1‐sensitised counterparts. However, non‐destructive splenic retention of G1Δnab‐coated RBCs required investigation and plasma radioactivities now suggest this also occurred for platelets sensitised with an IgG1/G1Δnab mixture. In vitro assays with human cells showed that G1Δnab‐sensitised RBCs did not cause FcγRI‐mediated monocyte activation, FcγRIIIa‐mediated antibody‐dependent cell‐mediated cytotoxicity (ADCC) or macrophage phagocytosis although they did adhere to macrophages. Thus, FcγRII was implicated in the adhesion despite the Δnab mutation reducing the already low‐affinity binding to this receptor class. Additional contacts via P‐selectin enhance the interaction of sensitised platelets with monocytes and this system provided evidence of FcγRII‐dependent activation by G1Δnab. These results emphasise the physiological relevance of low‐affinity interactions: It appears that FcγRII interactions of G1Δnab allowed splenic retention of G1Δnab‐coated RBCs with inhibitory FcγRIIb binding preventing RBC destruction and that FcγRIIb engagement by G1Δnab on IgG1/G1Δnab‐sensitised platelets overcame activation by IgG1. Considering therapeutic blocking Abs, G1Δnab offers lower FcγR binding and a greater bias towards inhibition than IgG2 and IgG4 constant regions.     

Fixing an irrelevant TCRα chain reveals the importance of TCRβ diversity for optimal TCRαβ pairing and function of virus‐specific CD8+ T cells

TCR repertoire diversity can influence the efficacy of CD8⁺ T‐cell populations, with greater breadth eliciting better protection. We analyzed TCRβ diversity and functional capacity for influenza‐specific CD8⁺ T cells expressing a single TCRα chain. Mice (A7) transgenic for the H2K^bOVA_257–264‐specific Vα2.7 TCR were challenged with influenza to determine how fixing this “irrelevant” TCRα affects the “public” and restricted D^bNP ${}_{\text{366}}^{\text{+}}$CD8⁺ versus the “private” and diverse D^bPA ${}_{\text{224}}^{\text{+}}$CD8⁺ responses. Though both D^bNP ${}_{\text{366}}^{\text{+}}$CD8⁺ and D^bPA ${}_{\text{224}}^{\text{+}}$CD8⁺ sets are generated in virus‐primed A7 mice, the constrained D^bNP ${}_{\text{366}}^{\text{+}}$CD8⁺ population lacked the characteristic, public TCRVβ8.3, and consequently was reduced in magnitude and pMHC‐I avidity. For the more diverse D^bPA ${}_{\text{224}}^{\text{+}}$CD8⁺ T cells, this particular forcing led to a narrowing and higher TCRβ conservation of the dominant Vβ7, though the responses were of comparable magnitude to C57BL/6J controls. Interestingly, although both the TCRβ diversity and the cytokine profiles were reduced for the D^bNP ${}_{\text{366}}^{\text{+}}$CD8⁺ and D^bPA ${}_{\text{224}}^{\text{+}}$CD8⁺ sets in spleen, the latter measure of polyfunctionality was comparable for T cells recovered from the infected lungs of A7 and control mice. Even “sub‐optimal” TCRαβ pairs can operate effectively when exposed in a milieu of high virus load. Thus, TCRβ diversity is important for optimal TCRαβ pairing and function when TCRα is limiting.     

Co-expression of CD8α in CD4+ T cell receptor αβ+ T cells migrating into the murine small intestine epithelial layer

We investigated the surface phenotype of CD3⁺CD4⁺ T cell receptor (TCR) αβ⁺ T cells repopulating the intestinal lymphoid tissues of C.B-17 scidlscid (severe-combined immunodeficient; scid) (H-2^d, L^d+) mice. CD4⁺ CD8^? T cells were cell sorter-purified from various secondary and tertiary lymphoid organs of congenic C.B-17 +/+ (H-2^d, L^d+) or semi-syngeneic dm2 (H-2^d, L^d?) immunocompetent donor mice. After transfer of 10⁵ cells into young scid mice, a mucosa-homing, memory CD44^hi CD45RB^lo CD4⁺ T cell population was selectively engrafted. Large numbers of single-positive (SP) CD3⁺ CD2⁺ CD28⁺ CD4⁺ CD^8? T cells that expressed the α₄ integrin chain CD49d were found in the spleen, the mesenteric lymph nodes, the peritoneal cavity and the gut lamina propria of transplanted scid mice. Unexpectedly, large populations of donor-type doublepositive (DP) CD4⁺ CD8α⁺ CD8β^? T cells with high expression of the CD3/TCR complex appeared in the epithelial layer of the small intestine of transplanted scid mice. In contrast to SP CD4⁺ T cells, the intraepithelial DP T cells showed low expression of the CD2 and the CD28 co-stimulator molecules, and of the α₄ integrin chain CD49d, but expressed high levels of the α_IEL integrin chain CD103. The TCR-Vβ repertoire of DP but not SP intraepithelial CD4⁺ T cells was biased towards usage of the Vβ6 and Vβ8 viable domains. Highly purified populations of SP and DP CD4⁺ T cell populations from the small intestine epithelial layer of transplanted scid mice had different abilities to repopulate secondary scid recipient mice: SP CD4⁺ T cells repopulated various lymphoid tissues of the immunodeficient host, while intraepithelial DP CD4⁺ T cells did not. Hence, a subset of CD3⁺ CD4⁺ TCRαβ⁺ T cells apparently undergoes striking phenotypic changes when it enters the microenvironment of the small intestine epithelial layer.     

CD40L expression by CD4+ but not CD8+ T cells regulates antiviral immune responses in acute LCMV infection in mice

CD40‐CD40 ligand (CD40L) signaling plays multiple indispensable roles in cellular and humoral immunity. Impaired memory T‐cell responses in the absence of CD40L have been well documented, but the requirement of this interaction for efficient priming of CD8⁺ T cells especially under inflammatory conditions has been under debate. In contrast to previous publications, we report here that virus‐specific CD8⁺ T‐cell responses as well as viral clearance are affected not only in the memory but also in the effector phase in CD40L^?/? mice infected with lymphocytic choriomeningitis virus (LCMV) Armstrong strain. Interestingly, a considerable part of the LCMV‐specific effector and memory T cells consists of CD40L⁺ CD8⁺ T cells. However, deficiency of CD40L in CD8⁺ T cells did influence neither the quantity nor the quality of primary T‐cell responses in LCMV infection. Virus‐specific CD8⁺ T cells in conditional knockout mice, with a selective deletion of the CD40L in CD8⁺ T cells, were fully functional regarding cytokine production and efficient pathogen clearance. Thus, our results unambiguously demonstrate that while CD40L is critical to generate effective primary CD8⁺ T‐cell responses also under inflammatory conditions, CD40L expression by CD8⁺ T cells themselves is dispensable in acute LCMV infection.     

Both FcγRIV and FcγRIII are essential receptors mediating type II and type III autoimmune responses via FcRγ‐LAT‐dependent generation of C5a

FcγRIV is a relatively new IgG Fc receptor (FcγR) that is reported to contribute to the pathogenesis of autoimmune diseases, although its specific role in relation to FcγRIII, complement and IgG2 subclasses remains uncertain. Here we define FcγRIV on macrophages as a receptor for soluble IgG2a/b complexes but not for cellular bound IgG2a and show that simultaneous activation of FcγRIV and FcγRIII is critical to mediate certain type II/III autoimmune responses. FcγRIII‐deficient mice display compensatory enhanced FcγRIV expression, are protected from lung inflammation after deposition of IgG complexes, and show reduced sensitivity to IgG2a/b‐mediated hemolytic anemia, indicating that increased FcγRIV alone is not sufficient to trigger these diseases in the absence of FcγRIII. Importantly, however, blockade of FcγRIV is also effective in inhibiting phagocytosis and cytokine production in IgG2b‐induced anemia and acute lung injury, processes that display a further dependence on C5a anaphylatoxin receptor. Using gene deletion and functional inhibition studies, we found that FcγRIII and FcγRIV are each essential to trigger an FcRγ‐linker for activation of T‐cell‐dependent signal that drives C5a production in the Arthus reaction. Together, the results demonstrate a combined requirement for FcγRIII and FcγRIV in autoimmune injury, and identify the linker for activation of T cells adaptor as an integral component of linked FcγR and C5a anaphylatoxin receptor activation to generate inflammation.     

Biphasic role of 4‐1BB in the regulation of mouse cytomegalovirus‐specific CD8+ T cells

The initial requirement for the emergence of CMV‐specific CD8⁺ T cells is poorly understood. Mice deficient in the cosignaling TNF superfamily member, 4‐1BB, surprisingly developed exaggerated early CD8⁺ T‐cell responses to mouse CMV (MCMV). CD8⁺ T cells directed against acute MCMV epitopes were enhanced, demonstrating that 4‐1BB naturally antagonizes these primary populations. Paradoxically, 4‐1BB‐deficient mice displayed reduced accumulation of memory CD8⁺ T cells that expand during chronic/latent infection. Importantly, the canonical TNF‐related ligand, 4‐1BBL, promoted the accumulation of these memory CD8⁺ T cells, whereas suppression of acute CD8⁺ T cells was independent of 4‐1BBL. These data highlight the dual nature of the 4‐1BB/4‐1BBL system in mediating both stimulatory and inhibitory cosignaling activities during the generation of anti‐MCMV immunity.     

Activation‐induced CD137 is a fast assay for identification and multi‐parameter flow cytometric analysis of alloreactive T cells

Detection and isolation of viable alloreactive T cells at the single‐cell level requires a cell surface marker induced specifically upon T cell receptor activation. In this study, a member of the tumour necrosis factor receptor (TNFR)‐family, CD137 (4‐1BB) was investigated for its potential to identify the total pool of circulating alloreactive T cells. Optimal conditions for sensitive and specific detection of allogeneic‐induced CD137 expression on circulating T cells were established. Thereafter, CD137⁺ alloreactive T cells were phenotypically and functionally characterized by multi‐parameter flow cytometry. Alloantigen‐induced CD137 expression identified both alloreactive CD8⁺ T cells (mean ± standard error of the mean: 0·21 ± 0·07%) and alloreactive CD4⁺ T cells (0·21 ± 0·05%). CD137⁺ alloreactive T cells were detected in different T cell subsets, including naive T cells, but were found preferentially in CD28⁺ T cells and not in the terminally differentiated T cell subset. Upon allogeneic (re‐)stimulation, the cytokine‐producing as well as proliferative capacity of T cells resided mainly within the CD137‐expressing fraction. About 10% of the CD137⁺ alloreactive T cells produced any combination of interferon (IFN)‐γ, interleukin (IL)‐2 and TNF‐α. Polyfunctional alloreactive T cells, defined by multiple cytokine expression, were observed infrequently. In conclusion, activation‐induced CD137 expression is a fast assay allowing for detection and functional analysis of the total alloreactive T cell compartment at the single‐cell level by multi‐parameter flow cytometry.     

Neonates harbour highly active γδ T cells with selective impairments in preterm infants

Acknowledgement of the breadth of T‐cell pleiotropy has provoked increasing interest in the degree to which functional responsiveness is elicited by environmental cues versus differentiation. This is particularly relevant for young animals requiring rapid responses to acute environmental exposure. In young mice, γδ T cells are disproportionately important for immuno‐protection. To examine the situation in humans, we compared populations and clones of T cells from term and preterm babies, and adults. By comparison with αβ T cells, neonate‐derived γδ cells show stronger, pleiotropic functional responsiveness, and lack signatory deficits in IFN‐γ production. Emphasising the acquisition of functional competence in utero, IFN‐γ was produced by γδ cells sampled from premature births, and, although one month's post‐partum environmental exposure invariably increased their TNF‐α production, it had no consistent effect on IFN‐γ or IL‐2. In sum, γδ cells seem well positioned at birth to contribute to immuno‐protection and immuno‐regulation, possibly compensating for selective immaturity in the αβ compartment. With regard to the susceptibilities of preterm babies to viral infection, γδ cells from preterm neonates were commonly impaired in Toll‐like receptor‐3 and ‐7 expression and compared with cells from term babies failed to optimise cytokine production in response to coincident TCR and TLR agonists.     

Card9 controls Dectin‐1‐induced T‐cell cytotoxicity and tumor growth in mice

Activation of the C‐type lectin receptor Dectin‐1 by β‐glucans triggers multiple signals within DCs that result in activation of innate immunity. While these mechanisms can potently prime CD8⁺ cytotoxic T‐cell (CTL) responses without additional adjuvants, the Dectin‐1 effector pathways that control CTL induction remain unclear. Here we demonstrate that Dectin‐1‐induced CTL cross‐priming in mice does not require inflammasome activation but strictly depends on the adapter protein Card9 in vitro. In vivo, Dectin‐1‐mediated Card9 activation after vaccination drives both expansion and activation of Ag‐specific CTLs, resulting in long‐lasting CTL responses that are sufficient to protect mice from tumor challenge. This Dectin‐1‐induced antitumor immune response was independent of NK cell function and completely abrogated in Card9‐deficient mice. Thus, our results demonstrate that Dectin‐1‐triggered Card9 signaling but not inflammasome activation can potently cross‐prime Ag‐specific CTLs, suggesting that this pathway would be a candidate for immunotherapy and vaccine development.     

Interrupting CD28 costimulation before antigen rechallenge affects CD8+ T‐cell expansion and effector functions during secondary response in mice

The role of CD28‐mediated costimulation in secondary CD8⁺ T‐cell responses remains controversial. Here, we have used two tools — blocking mouse anti‐mouse CD28‐specific antibodies and inducible CD28‐deleting mice — to obtain definitive answers in mice infected with ovalbumin‐secreting Listeria monocytogenes. We report that both blockade and global deletion of CD28 reveal its requirement for full clonal expansion and effector functions such as degranulation and IFN‐γ production during the secondary immune response. In contrast, cell‐intrinsic deletion of CD28 in transferred TCR‐transgenic CD8⁺ T cells before primary infection leads to impaired clonal expansion but an increase in cells able to express effector functions in both primary and secondary responses. We suggest that the proliferation‐impaired CD8⁺ T cells respond to CD28‐dependent help from their environment by enhanced functional differentiation. Finally, we report that cell‐intrinsic deletion of CD28 after the peak of the primary response does not affect the establishment, maintenance, or recall of long‐term memory. Thus, if given sufficient time, the progeny of primed CD8⁺ T cells adapt to the absence of this costimulator.     

Altered effector functions of virus‐specific and virus cross‐reactive CD8+ T cells in mice immunized with related flaviviruses

Memory cross‐reactive CD8⁺ T‐cell responses may induce protection or immunopathology upon secondary viral challenge. To elucidate the potential role of T cells in sequential flavivirus infection, we characterized cross‐reactive CD4⁺ and CD8⁺ T‐cell responses between attenuated and pathogenic Japanese encephalitis virus (JEV) and pathogenic West Nile virus (WNV). A previously reported WNV NS4b CD8⁺ T‐cell epitope and its JEV variant elicited CD8⁺ T‐cell responses in both JEV‐ and WNV‐infected mice. The peptide variant homologous to the immunizing virus induced greater cytokine secretion and activated higher frequencies of epitope‐specific CD8⁺ T cells. However, there was a virus‐dependent, peptide variant‐independent pattern of cytokine secretion; the IFNγ⁺‐to‐IFNγ⁺TNFα⁺ CD8⁺ T‐cell ratio was greater in JEV‐ than in WNV‐infected mice. Despite similarities in viral burden for pathogenic WNV and JEV viruses, CD8⁺ T cells from pathogenic JEV‐immunized mice exhibited functional and phenotypic profiles similar to those seen for the attenuated JEV strain. Patterns of killer cell lectin‐like receptor G1 (KLRG1) and CD127 expression differed by virus type, with a rapid expansion and contraction of short‐lived effector cells in JEV infection and persistence of high levels of short‐lived effector cells in WNV infection. Such cross‐reactive T‐cell responses to primary infection may affect the outcomes of sequential flavivirus infections.