Clonal competition inhibits the proliferation and differentiation of adoptively transferred TCR transgenic CD4 T cells in response to infection

How persistent infection overcomes peripheral tolerance mechanisms to cause T cell-mediated autoimmune disease

T cells help orchestrate immune responses to pathogens, and their aberrant regulation can trigger autoimmunity. Recent studies highlight that a threshold number of T cells (a quorum) must be activated in a tissue to mount a functional immune response. These collective effects allow the T cell repertoire to respond to pathogens while suppressing autoimmunity due to circulating autoreactive T cells. Our computational studies show that increasing numbers of pathogenic peptides targeted by T cells during persistent or severe viral infections increase the probability of activating T cells that are weakly reactive to self-antigens (molecular mimicry). These T cells are easily re-activated by the self-antigens and contribute to exceeding the quorum threshold required to mount autoimmune responses. Rare peptides that activate many T cells are sampled more readily during severe/persistent infections than in acute infections, which amplifies these effects. Experiments in mice to test predictions from these mechanistic insights are suggested.

Egr2 and 3 control inflammation, but maintain homeostasis, of PD-1high memory phenotype CD4 T cells

PD-1^high memory CD4 T cells are pathogenic in autoimmune disease; here they show their expression of Egr2 is defective in rheumatoid arthritis and Egr2 & 3 control their inflammation and homeostasis.

The transcription factors Egr2 and 3 are essential for controlling inflammatory autoimmune responses of memory phenotype (MP) CD4 T cells. However, the mechanism is still unclear. We have now found that the Egr2⁺ subset (PD-1^high MP) of MP CD4 T cells expresses high levels of checkpoint molecules (PD-1 and Lag3) and also markers of effector T cells (CXCR3 and ICAM-1). Egr2/3 are not required for PD-1^high MP CD4 cell development but mediate a unique transcriptional programme that effectively controls their inflammatory responses, while promoting homeostatic proliferation and adaptive responses. Egr2 negative PD-1^high MP CD4 T cells are impaired in homeostatic proliferation and adaptive responses against viral infection but display inflammatory responses to innate stimulation such as IL-12. PD-1^high MP CD4 T cells have recently been implicated in rheumatoid arthritis pathogenesis, and we have now found that Egr2 expression is reduced in PD-1^high MP CD4 T cells from patients with active rheumatoid arthritis compared with healthy controls. These findings demonstrate that Egr2/3 control the inflammatory responses of PD-1^high MP CD4 T cells and maintain their adaptive immune fitness.

Memory CD4+ T Cells in Immunity and Autoimmune Diseases

CD4⁺ T helper (Th) cells play central roles in immunity in health and disease. While much is known about the effector function of Th cells in combating pathogens and promoting autoimmune diseases, the roles and biology of memory CD4⁺ Th cells are complex and less well understood. In human autoimmune diseases such as multiple sclerosis (MS), there is a critical need to better understand the function and biology of memory T cells. In this review article we summarize current concepts in the field of CD4⁺ T cell memory, including natural history, developmental pathways, subsets, and functions. Furthermore, we discuss advancements in the field of the newly-described CD4⁺ tissue-resident memory T cells and of CD4⁺ memory T cells in autoimmune diseases, two major areas of important unresolved questions in need of answering to advance new vaccine design and development of novel treatments for CD4⁺ T cell-mediated autoimmune diseases.

Neuroinflammation-induced lymphangiogenesis near the cribriform plate contributes to drainage of CNS-derived antigens and immune cells

There are no conventional lymphatic vessels within the CNS parenchyma, although it has been hypothesized that lymphatics near the cribriform plate or dura maintain fluid homeostasis and immune surveillance during steady-state conditions. However, the role of these lymphatic vessels during neuroinflammation is not well understood. We report that lymphatic vessels near the cribriform plate undergo lymphangiogenesis in a VEGFC – VEGFR3 dependent manner during experimental autoimmune encephalomyelitis (EAE) and drain both CSF and cells that were once in the CNS parenchyma. Lymphangiogenesis also contributes to the drainage of CNS derived antigens that leads to antigen specific T cell proliferation in the draining lymph nodes during EAE. In contrast, meningeal lymphatics do not undergo lymphangiogenesis during EAE, suggesting heterogeneity in CNS lymphatics. We conclude that increased lymphangiogenesis near the cribriform plate can contribute to the management of neuroinflammation-induced fluid accumulation and immune surveillance.

Lymphangiogenesis occurs in the context of systemic inflammation and development but has not been reported for the lymphatics that surround the CNS. Here the authors show that in the context of experimental autoimmune encephatlitis, lymphangiogenesis occurs at the cribriform plate, but not the meninges, and contributes to immune cell and antigen drainage.

Early programming and late-acting checkpoints governing the development of CD4 T-cell memory

CD4 T cells contribute to protection against pathogens through numerous mechanisms. Incorporating the goal of memory CD4 T-cell generation into vaccine strategies therefore offers a powerful approach to improve their efficacy, especially in situations where humoral responses alone cannot confer long-term immunity. These threats include viruses such as influenza that mutate coat proteins to avoid neutralizing antibodies, but that are targeted by T cells that recognize more conserved protein epitopes shared by different strains. A major barrier in the design of such vaccines is that the mechanisms controlling the efficiency with which memory cells form remain incompletely understood. Here, we discuss recent insights into fate decisions controlling memory generation. We focus on the importance of three general cues: interleukin-2, antigen and co-stimulatory interactions. It is increasingly clear that these signals have a powerful influence on the capacity of CD4 T cells to form memory during two distinct phases of the immune response. First, through 'programming' that occurs during initial priming, and second, through 'checkpoints' that operate later during the effector stage. These findings indicate that novel vaccine strategies must seek to optimize cognate interactions, during which interleukin-2-, antigen- and co-stimulation-dependent signals are tightly linked, well beyond initial antigen encounter to induce robust memory CD4 T cells.

Monitoring and Modulation of Inducible Foxp3+ Regulatory T-Cell Differentiation in the Lymph Nodes Draining the Small Intestine and Colon

The mucosa-draining lymphoid tissue favors differentiation of inducible Foxp3⁺ regulatory T cells. Adoptive transfer of T-cell receptor (TCR) transgenic (Tg) T cells is a powerful tool to study antigen-specific regulatory T-cell differentiation in lymphoid tissues in vivo. The kinetics and nature of the T-cell response largely depend on the route of antigen administration and degree of clonal competition. Here, we describe that adoptive transfer of CD4⁺ DO11.10 TCR Tg T cells can be used for monitoring Foxp3⁺ regulatory T-cell differentiation in the gut-draining lymph nodes. We describe two routes of mucosal antigen administration, e.g., the oral and intracolonic route known to induce T-cell responses in the small intestine-draining mesenteric lymph nodes (MLN) and distal colon-draining caudal and iliac lymph nodes (ILN), respectively. In particular, we discuss differences in frequency of inducible Foxp3⁺ regulatory T cells after adoptive transfer of variable numbers of Tg T cells and various amounts of orally gavaged ovalbumin (OVA), and explain how Foxp3⁺ regulatory T-cell differentiation can be modulated by coadministration of the adjuvant cholera toxin (CT) with OVA using this adoptive transfer system.

Competition within the virus-specific CD4 T-cell pool limits the T follicular helper response after influenza infection

CD4 T follicular helper cells (TFH) are critical in the generation of potent and long-lived B-cell responses after viral infection. However, the factors that dictate the generation and maintenance of these cells are not fully understood. Here we use adoptive transfer of OTII T-cell receptor transgenic CD4 T cells, followed by infection with recombinant influenza A virus (IAV), as a means of identifying and tracking virus-specific CD4(+) T-cell responses. We show that T-cell competition within the virus-specific CD4 T-cell pool induced by IAV infection limits the proliferation and differentiation of IAV-specific CD4(+) TFH responses. In particular, increased T-cell competition for antigen results in a diminished IAV-specific TFH CD4 T-cell responses, particularly germinal center TFH responses. Strikingly, competition in the form of preexisting cellular immunity generated by heterosubtypic IAV immunization limits de novo CD4 T-cell responses in secondary lymphoid tissue. Taken together, these data show a profound linkage between antigen availability and promotion of TFH CD4(+) T-cell responses in response to infection. These data suggest that competition within the CD4 T-cell pool limits TFH responses and may be an important regulatory mechanism for controlling immunity.

Clonal Abundance of Tumor-Specific CD4(+) T Cells Potentiates Efficacy and Alters Susceptibility to Exhaustion

Current approaches to cancer immunotherapy aim to engage the natural T cell response against tumors. One limitation is the elimination of self-antigen-specific T cells from the immune repertoire. Using a system in which precursor frequency can be manipulated in a murine melanoma model, we demonstrated that the clonal abundance of CD4(+) T cells specific for self-tumor antigen positively correlated with antitumor efficacy. At elevated precursor frequencies, intraclonal competition impaired initial activation and overall expansion of the tumor-specific CD4(+) T cell population. However, through clonally derived help, this population acquired a polyfunctional effector phenotype and antitumor immunity was enhanced. Conversely, development of effector function was attenuated at low precursor frequencies due to irreversible T cell exhaustion. Our findings assert that the differential effects of T cell clonal abundance on phenotypic outcome should be considered during the design of adoptive T cell therapies, including use of engineered T cells.

Phenotype and Hierarchy of Two Transgenic T Cell Lines Targeting the Respiratory Syncytial Virus KdM282-90 Epitope Is Transfer Dose-Dependent

In this study, we compared two lines of transgenic CD8+ T cells specific for the same K^dM2_82-90 epitope of respiratory syncytial virus in the CB6F1 hybrid mouse model. Here we found that these two transgenic lines had similar in vivo abilities to control viral load after respiratory syncytial virus infection using adoptive transfer. Transfer of the TRBV13-2 line resulted in higher levels of IL-6 and MIP1-α in the lung than TRBV13-1 transfer. Interestingly, when large numbers of cells were co-transferred, the lines formed a hierarchy, with TRBV13-2 being immunodominant over TRBV13-1 in the mediastinal lymph node despite no identifiable difference in proliferation or apoptosis between the lines. This hierarchy was not established when lower cell numbers were transferred. The phenotype and frequency of proliferating cells were also cell transfer dose-dependent with higher percentages of CD127^loCD62L^loKLRG1^lo and proliferating cells present when lower numbers of cells were transferred. These results illustrate the importance of cell number in adoptive transfer experiments and its influence on the phenotype and hierarchy of the subsequent T cell response.

Narrowing the Gap: Preserving Repertoire Diversity Despite Clonal Selection during the CD4 T Cell Response

T cell immunity relies on the generation and maintenance of a diverse repertoire of T cell antigen receptors (TCRs). The strength of signaling emanating from the TCR dictates the fate of T cells during development, as well as during the immune response. Whereas development of new T cells in the thymus increases the available TCR repertoire, clonal selection during the immune response narrows TCR diversity through the outgrowth of clonotypes with the fittest TCR. To ensure maintenance of TCR diversity in the antigen-selected repertoire, specific mechanisms can be envisaged that facilitate the participation of T cell clonotypes with less than best fit TCRs. Here, we summarize the evidence for the existence of such mechanisms that can prevent the loss of diversity. A number of T cell-autonomous or extrinsic factors can reverse clonotypic hierarchies set by TCR affinity for given antigen. Although not yet complete, understanding of these factors and their mechanism of action will be critical in interventional attempts to mold the antigen-selected TCR repertoire.

T follicular helper, but not Th1, cell differentiation in the absence of conventional dendritic cells

Development of long-lived humoral immunity is dependent on CXCR5-expressing T follicular helper (Tfh) cells, which develop concomitantly to effector Th cells that support cellular immunity. Conventional dendritic cells (cDCs) are critical APCs for initial priming of naive CD4(+) T cells but, importantly, also provide accessory signals that govern effector Th cell commitment. To define the accessory role of cDCs during the concurrent development of Tfh and effector Th1 cells, we performed high-dose Ag immunization in conjunction with the Th1-biased adjuvant polyinosinic:polycytidylic acid (pI:C). In the absence of cDCs, pI:C failed to induce Th1 cell commitment and IgG2c production. However, cDC depletion did not impair Tfh cell differentiation or germinal center formation, and long-lived IgG1 responses of unaltered affinity developed in mice lacking cDCs at the time point for immunization. Thus, cDCs are required for the pI:C-driven Th1 cell fate commitment but have no crucial accessory function in relation to Tfh cell differentiation.

Clonotypic composition of the CD4+ T cell response to a vectored retroviral antigen is determined by its speed

The mechanisms whereby different vaccines may expand distinct Ag-specific T cell clonotypes or induce disparate degrees of protection are incompletely understood. We found that several delivery modes of a model retroviral Ag, including natural infection, preferentially expanded initially rare high-avidity CD4(+) T cell clonotypes, known to mediate protection. In contrast, the same Ag vectored by human adenovirus serotype 5 induced clonotypic expansion irrespective of avidity, eliciting a predominantly low-avidity response. Nonselective clonotypic expansion was caused by relatively weak adenovirus serotype 5-vectored Ag presentation and was reproduced by replication-attenuated retroviral vaccines. Mechanistically, the potency of Ag presentation determined the speed and, consequently, completion of the CD4(+) T cell response. Whereas faster completion retained the initial advantage of high-avidity clonotypes, slower completion permitted uninhibited accumulation of low-avidity clonotypes. These results highlighted the importance of Ag presentation patterns in determining the clonotypic composition of vaccine-induced T cell responses and ultimately the efficacy of vaccination.

Dynamic functional modulation of CD4+ T cell recall responses is dependent on the inflammatory environment of the secondary stimulus

The parameters that modulate the functional capacity of secondary Th1 effector cells are poorly understood. In this study, we employ a serial adoptive transfer model system to show that the functional differentiation and secondary memory potential of secondary CD4+ effector T cells are dependent on the inflammatory environment of the secondary challenge. Adoptive transfer of TCR transgenic lymphocytic choriomeningitis virus (LCMV) Glycoprotein-specific SMARTA memory cells into LCMV-immune hosts, followed by secondary challenge with Listeria monocytogenes recombinantly expressing a portion of the LCMV Glycoprotein (Lm-gp61), resulted in the rapid emergence of SMARTA secondary effector cells with heightened functional avidity (as measured by their ability to make IFNγ in response to ex vivo restimulation with decreasing concentrations of peptide), limited contraction after pathogen clearance and stable maintenance secondary memory T cell populations. In contrast, transfer of SMARTA memory cells into naïve hosts prior to secondary Lm-gp61 challenge, which resulted in a more extended infectious period, resulted in poor functional avidity, increased death during the contraction phase and poor maintenance of secondary memory T cell populations. The modulation of functional avidity during the secondary Th1 response was independent of differences in antigen load or persistence. Instead, the inflammatory environment strongly influenced the function of the secondary Th1 response, as inhibition of IL-12 or IFN-I activity respectively reduced or increased the functional avidity of secondary SMARTA effector cells following rechallenge in a naïve secondary hosts. Our findings demonstrate that secondary effector T cells exhibit inflammation-dependent differences in functional avidity and memory potential, and have direct bearing on the design of strategies aimed at boosting memory T cell responses.

Effective T helper cell responses against retroviruses: are all clonotypes equal?

The critical importance of CD4(+) T cells in coordinating innate and adaptive immune responses is evidenced by the susceptibility to various pathogenic and opportunistic infections that arises from primary or acquired CD4(+) T cell immunodeficiency, such as following HIV-1 infection. However, despite the clearly defined roles of cytotoxic CD8(+) T cells and antibodies in host protection from retroviruses, the ability of CD4(+) T cells to exert a similar function remains unclear. Recent studies in various settings have drawn attention to the complexity of the T cell response within and between individuals. Distinct TCR clonotypes within an individual differ substantially in their response to the same epitope. Functionally similar, "public" TCR clonotypes can also dominate the response of different individuals. TCR affinity for antigen directly influences expansion and differentiation of responding T cells, also likely affecting their ultimate protective capacity. With this increasing understanding of the parameters that determine the magnitude and effector type of the T cell response, we are now better equipped to address the protective capacity against retroviruses of CD4(+) T cell clonotypes induced by natural infection or vaccination.

Sustained interactions between T cell receptors and antigens promote the differentiation of CD4⁺ memory T cells

During CD4⁺ T cell activation, T cell receptor (TCR) signals impact T cell fate, including recruitment, expansion, differentiation, trafficking, and survival. To determine the impact of TCR signals on the fate decision of activated CD4⁺ T cells to become end-stage effector or long-lived memory T helper 1 (Th1) cells, we devised a deep-sequencing-based approach that allowed us to track the evolution of TCR repertoires after acute infection. The transition of effector Th1 cells into the memory pool was associated with a significant decrease in repertoire diversity, and the major histocompatibility complex (MHC) class II tetramer off rate, but not tetramer avidity, was a key predictive factor in the representation of individual clonal T cell populations at the memory stage. We conclude that stable and sustained interactions with antigens during the development of Th1 responses to acute infection are a determinative factor in promoting the differentiation of Th1 memory cells.

Differential T cell responses to residual viral antigen prolong CD4+ T cell contraction following the resolution of infection

The contraction phase of the T cell response is a poorly understood period after the resolution of infection when virus-specific effector cells decline in number and memory cells emerge with increased frequencies. CD8(+) T cells plummet in number and quickly reach stable levels of memory following acute lymphocytic choriomeningitis virus infection in mice. In contrast, virus-specific CD4(+) T cells gradually decrease in number and reach homeostatic levels only after many weeks. In this study, we provide evidence that MHCII-restricted viral Ag persists during the contraction phase following this prototypical acute virus infection. We evaluated whether the residual Ag affected the cell division and number of virus-specific naive and memory CD4(+) T cells and CD8(+) T cells. We found that naive CD4(+) T cells underwent cell division and accumulated in response to residual viral Ag for >2 mo after the eradication of infectious virus. Surprisingly, memory CD4(+) T cells did not undergo cell division in response to the lingering Ag, despite their heightened capacity to recognize Ag and make cytokine. In contrast to CD4(+) T cells, CD8(+) T cells did not undergo cell division in response to the residual Ag. Thus, CD8(+) T cells ceased division within days after the infection was resolved, indicating that CD8(+) T cell responses are tightly linked to endogenous processing of de novo synthesized virus protein. Our data suggest that residual viral Ag delays the contraction of CD4(+) T cell responses by recruiting new populations of CD4(+) T cells.

Antigen-stimulated CD4 T cell expansion can be limited by their grazing of peptide-MHC complexes

It was recently shown that the expansion of CD4(+) T cells during a primary immune reaction to a peptide from cytochrome c decreases ~0.5 log for every log increase in the number of cognate precursor cells, and that this remains valid over more than four orders of magnitude (Quiel et al. 2011. Proc. Natl. Acad. Sci. USA. 108: 3312-3317). This observed "power law" was explained by a mechanism where nondividing mature T cells inhibit the proliferation of less-differentiated cells of the same specificity. In this article, we interpret the same data by a mechanism where CD4(+) T cells acquire cognate peptide-MHC (pMHC) complexes from the surface of APCs, thereby increasing the loss rate of pMHC. We show that a mathematical model implementing this "T cell grazing" mechanism, and having a T cell proliferation rate that is determined by the concentration of pMHC, explains the data equally well. As a consequence, the data no longer unequivocally support the previous explanation, and the increased loss of pMHC complexes on APCs at high T cell densities is an equally valid interpretation of this striking data.

Secondary T cell-T cell synaptic interactions drive the differentiation of protective CD8+ T cells

Immunization results in the differentiation of CD8+ T cells, such that they acquire effector abilities and convert into a memory pool. Priming of T cells takes place via an immunological synapse formed with an antigen-presenting cell (APC). By disrupting synaptic stability at different times, we found that the differentiation of CD8+ T cells required cell interactions beyond those made with APCs. We identified a critical differentiation period that required interactions between primed T cells. We found that T cell-T cell synapses had a major role in the generation of protective CD8+ T cell memory. T cell-T cell synapses allowed T cells to polarize critical secretion of interferon-γ (IFN-γ) toward each other. Collective activation and homotypic clustering drove cytokine sharing and acted as regulatory stimuli for T cell differentiation.

T cell responses to antigen: hasty proposals resolved through long engagements

T cells discriminate between peptide-MHC complexes on the surfaces of antigen presenting cells to enact appropriate downstream responses. Great progress has been made over the last 15 years in understanding varied aspects of T cell activation on short timescales (minutes), yet the mechanics and significance of long term T cell receptor signaling (hours or days) remain unclear. Furthermore, there remain some controversies regarding the correlation of the biophysical parameters of ligand-receptor interactions with the scaling of downstream effector functions. Here we review recent studies that emphasize the importance of long-term engagement of antigens to fine-tuning the activation of T cells over the duration of the complete immune response. We discuss how T cells dynamically regulate T cell receptor signaling via antigen crosstalk, competition and consumption to accurately counter antigenic challenges.

At low precursor frequencies, the T-cell response to chronic self-antigen results in anergy without deletion

The behavior of self-reactive T cells in the peripheral immune system has often been studied by following the fate of adoptively transferred antigen-specific T cells in antigen expressing mice. In most cases, after a period of expansion, such cells undergo a slow clonal deletion, accompanied by the onset of anergy and/or suppression in the remaining cells. Here, we demonstrate that at initial frequencies approaching those found in normal repertoires, it is possible to completely avoid deletion and still maintain peripheral tolerance. At starting numbers of <1000 T cells, stimulation by chronic self-antigens resulted in a period of robust clonal expansion, followed by a steady plateau phase extending beyond 4 months. Despite their stable persistence, the self-reactive T cells did not convert to a Foxp3⁺ fate. However, they displayed a considerable block in their ability to make IL-2, consistent with the onset of anergy - in a precursor frequency or deletion independent fashion.

Homeostatic division is not necessary for antigen-specific CD4+ memory T cell persistence

CD4(+) memory T cells are generated in response to infection or vaccination, provide protection to the host against reinfection, and persist through a combination of enhanced survival and slow homeostatic turnover. We used timed deletion of the TCR-signaling adaptor molecule Src homology 2 domain-containing phosphoprotein of 76 kDa (SLP-76) with MHC:peptide tetramers to study the requirements for tonic TCR signals in the maintenance of polyclonal Ag-specific CD4(+) memory T cells. SLP-76-deficient I-A(b):gp61 cells are unable to rapidly generate effector cytokines or proliferate in response to secondary infection. In mice infected with lymphocytic choriomeningitis virus (LCMV) or Listeria monocytogenes expressing the LCMV gp61-80 peptide, SLP-76-deficient I-A(b):gp61(+) cells exhibit reduced division, similar to that seen in in vitro-generated CD44(hi) and endogenous CD4(+)CD44(hi) cells. Competitive bone marrow chimera experiments demonstrated that the decrease in homeostatic turnover in the absence of SLP-76 is a cell-intrinsic process. Surprisingly, despite the reduction in turnover, I-A(b):gp61(+) Ag-specific memory cells persist in normal numbers for >30 wk after LCMV infection in the absence of SLP-76. These data suggest the independent maintenance of a population of Ag-specific CD4(+) memory T cells in the absence of SLP-76 and normal levels of homeostatic division.

TCR signaling requirements for activating T cells and for generating memory

Over the last two decades the molecular and cellular mechanisms underlying T cell activation, expansion, differentiation, and memory formation have been intensively investigated. These studies revealed that the generation of memory T cells is critically impacted by a number of factors, including the magnitude of the inflammatory response and cytokine production, the type of dendritic cell [DC] that presents the pathogen derived antigen, their maturation status, and the concomitant provision of costimulation. Nevertheless, the primary stimulus leading to T cell activation is generated through the T cell receptor [TCR] following its engagement with a peptide MHC ligand [pMHC]. The purpose of this review is to highlight classical and recent findings on how antigen recognition, the degree of TCR stimulation, and intracellular signal transduction pathways impact the formation of effector and memory T cells.

The activation, by antigen, of naïve TCR transgenic CD4 T cells cultured at physiological, rather than artificially high, frequencies more accurately reflects the in vivo activation of normal numbers of naïve CD4(+) T cells

The majority of in vitro studies investigating the activation of naïve TCR transgenic T cells routinely employ an artificially high frequency of such cells. To assess whether employing high frequencies of TCR transgenic cells in vitro accurately reflects the in vivo activation of a normal number of T cells, we cultured between 300 and 3×10(6) Rag2(-/-) DO11.10 T cells per well under otherwise identical conditions. We find that those T cells cultured at low frequencies proliferate more and are more potently activated, as assessed by the expression of CD44 and CD62L, each giving rise to a much larger number of cytokine producing cells, comparable to the number generated in vivo when a normal number of CD4(+) T cells are activated. The effect of T cell frequency on the level of their activation was not due to differences in MHCII or CD80/86 expression by B cells, the major APC population present, nor to increased death of B cells in high frequency cultures. Taken together, our observations illustrate the necessity of culturing naïve TCR transgenic CD4(+) T cells at a physiological frequency if one is to more accurately recapitulate the in vivo activation of naïve CD4(+) T cells.

Origins of CD4(+) effector and central memory T cells

Lineage-committed effector CD4(+) T cells are generated at the peak of the primary response and are followed by heterogeneous populations of central and effector memory cells. Here we review the evidence that T helper type 1 (T(H)1) effector cells survive the contraction phase of the primary response and become effector memory cells. We discuss the applicability of this idea to the T(H)2 cell, T(H)17 helper T cell, follicular helper T cell (T(FH) cell) and induced regulatory T cell lineages. We also discuss how central memory cells are formed, with an emphasis on the role of B cells in this process.

CD4+ memory T cell survival

Memory CD4+ T cells specific for a given antigen are generated during the primary response from the effector lymphoblast progeny of naïve precursors. How memory CD4+ T cells differentiate from the effector population is not understood but new tools to assess transcription factor and cytokine expression are allowing for a more careful assessment of this process. Here we review the factors that allow some effector CD4+ T cells to survive the contraction phase of the primary response and become memory cells, and consider whether parallels can be drawn between T and B cells.

Antigen-stimulated CD4 T-cell expansion is inversely and log-linearly related to precursor number

Antigen-driven expansion of specific CD4 T cells diminishes, on a per cell basis, as infused cell number increases. There is a linear relation between log precursor number and log factor of expansion (FE), with a slope of ∼-0.5 over a range from 3 to 30,000 precursors. Cell number dependence of FE is observed at low precursor number, implying that the underlying process physiologically regulates antigen-driven T-cell expansion. FE of small numbers of transgenic precursors is not significantly affected by concomitant responses of large numbers of cells specific for different antigens. Increasing antigen amount or exogenous IL-2, IL-7, or IL-15 does not significantly affect FE, nor does FE depend on Fas, TNF-α receptor, cytotoxic T-lymphocyte antigen-4, IL-2, or IFN-γ. Small numbers of Foxp3-deficient T-cell receptor transgenic cells expand to a greater extent than do large numbers, implying that this effect is not mediated by regulatory T cells. Increasing dendritic cell number does result in larger FE, but the quantitative relation between FE and precursor number is not abrogated. Although not excluding competition for peptide/MHC complexes as an explanation, fall in FE with increasing precursor number could be explained by a negative feedback in which increasing numbers of responding cells in a cluster inhibit the expansion of cells of the same specificity within that cluster.

Feedback regulation of proliferation vs. differentiation rates explains the dependence of CD4 T-cell expansion on precursor number

The mechanisms regulating clonal expansion and contraction of T cells in response to immunization remain to be identified. A recent study established that there was a log-linear relation between CD4 T-cell precursor number (PN) and factor of expansion (FE), with a slope of ∼-0.5 over a range of 3-30,000 precursors per mouse. The results suggested inhibition of precursor expansion either by competition for specific antigen-presenting cells or by the action of other antigen-specific cells in the same microenvironment as the most likely explanation. Several molecular mechanisms potentially accounting for such inhibition were examined and rejected. Here we adopt a previously proposed concept, "feedback-regulated balance of growth and differentiation," and show that it can explain the observed findings. We assume that the most differentiated effectors (or memory cells) limit the growth of less differentiated effectors, locally, by increasing the rate of differentiation of the latter cells in a dose-dependent manner. Consequently, expansion is blocked and reversed after a delay that depends on initial PN, accounting for the dependence of the peak of the response on that number. We present a parsimonious mathematical model capable of reproducing immunization response kinetics. Model definition is achieved in part by requiring consistency with available BrdU-labeling and carboxyfluorescein diacetate succinimidyl ester (CFSE)-dilution data. The calibrated model correctly predicts FE as a function of PN. We conclude that feedback-regulated balance of growth and differentiation, although awaiting definite experimental characterization of the hypothetical cells and molecules involved in regulation, can explain the kinetics of CD4 T-cell responses to antigenic stimulation.

Induction and function of virus-specific CD4+ T cell responses

CD4+ T cells - often referred to as T-helper cells - play a central role in immune defense and pathogenesis. Virus infections and vaccines stimulate and expand populations of antigen-specific CD4+ T cells in mice and in man. These virus-specific CD4+ T cells are extremely important in antiviral protection: deficiencies in CD4+ T cells are associated with virus reactivation, generalized susceptibility to opportunistic infections, and poor vaccine efficacy. As described below, CD4+ T cells influence effector and memory CD8+ T cell responses, humoral immunity, and the antimicrobial activity of macrophages and are involved in recruiting cells to sites of infection. This review summarizes a few key points about the dynamics of the CD4+ T cell response to virus infection, the positive role of pro-inflammatory cytokines in the differentiation of virus-specific CD4+ T cells, and new areas of investigation to improve vaccines against virus infection.

Reduced functional avidity promotes central and effector memory CD4 T cell responses to tumor-associated antigens

The effect of TCR signals on the differentiation of memory T cells is poorly defined. Conventional wisdom suggests that high-avidity interactions are best for the selection of vaccine Ag candidates or T cell specificities for adoptive T cell therapy to stimulate robust responses. However, in conditions of Ag persistence, high-avidity clones might exhaust and fail to form long-lived protective memory. We have manipulated the functional avidity of CD4 T cells by reducing expression of Lck, a key kinase involved in TCR triggering. Using a mouse model, we followed tetramer-positive T cells responding to a tumor Ag expressed by an adenocarcinoma. We show that reducing the functional avidity increased effector-effector memory responses and improved the generation of self-renewing, recirculating, tumor Ag-specific memory phenotype CD4 T cells. Moreover, such cells together with wild type CD8 T cells were better able to control tumor growth. Mechanistically, reducing Lck prolonged IL-2 production and cell turnover in the central memory population while reducing expression of exhaustion markers in the face of chronic Ag. Our data indicate that, in situations of persistent Ag challenge, generating T cells with reduced functional avidity may elicit more effective immune responses.

The persistence of T cell memory

T cell memory is a crucial feature of the adaptive immune system in the defense against pathogens. During the last years, numerous studies have focused their efforts on uncovering the signals, inflammatory cues, and extracellular factors that support memory differentiation. This research is beginning to decipher the complex gene network that controls memory programming. However, how the different signals, that a T cell receives during the process of differentiation, interplay to trigger memory programming is still poorly defined. In this review, we focus on the most recent advances in the field and discuss how T cell receptor signaling and inflammation control CD8 memory differentiation.

Coxsackievirus B3 inhibits antigen presentation in vivo, exerting a profound and selective effect on the MHC class I pathway

Many viruses encode proteins whose major function is to evade or disable the host T cell response. Nevertheless, most viruses are readily detected by host T cells, and induce relatively strong T cell responses. Herein, we employ transgenic CD4⁺ and CD8⁺ T cells as sensors to evaluate in vitro and in vivo antigen presentation by coxsackievirus B3 (CVB3), and we show that this virus almost completely inhibits antigen presentation via the MHC class I pathway, thereby evading CD8⁺ T cell immunity. In contrast, the presentation of CVB3-encoded MHC class II epitopes is relatively unencumbered, and CVB3 induces in vivo CD4⁺ T cell responses that are, by several criteria, phenotypically normal. The cells display an effector phenotype and mature into multi-functional CVB3-specific memory CD4⁺ T cells that expand dramatically following challenge infection and rapidly differentiate into secondary effector cells capable of secreting multiple cytokines. Our findings have implications for the efficiency of antigen cross-presentation during coxsackievirus infection.

Many viruses—for example, large DNA viruses like smallpox virus and herpesviruses—encode several proteins whose major function is to combat the host's immune response, but these proteins usually battle in vain; in general, the mammalian immune system is sufficiently accomplished to penetrate this viral armor, allowing the infected animal to mount an immune response that can eradicate—or, at least, suppress—the infectious agent. Here, we show that coxsackievirus, a small RNA virus, carries a far more powerful punch than its larger DNA cousins; it almost entirely evades detection by host CD8⁺ T cells, which usually are one of the key components of an antiviral immune response. How does the virus achieve such success? Normally, when a virus infects a cell, certain host proteins capture small fragments of the virus and display them on the cell's surface, allowing them to be detected by the host immune system—usually, by cells called CD8⁺ T cells. We show here that coxsackievirus very effectively prevents these “flags” from reaching the cell surface in a form that can trigger naïve T cells to respond; in effect, the virus renders the cell “invisible” to CD8⁺ T cells, creating a cocoon in which the virus can multiply undisturbed by host immunity.

The magnitude of CD4+ T cell recall responses is controlled by the duration of the secondary stimulus

The parameters controlling the generation of robust CD4(+) T cell recall responses remain poorly defined. In this study, we compare recall responses by CD4(+) and CD8(+) memory T cells following rechallenge. Homologous rechallenge of mice immune to either lymphocytic choriomeningitis virus or Listeria monocytogenes results in robust CD8(+) T cell recall responses but poor boosting of CD4(+) T cell recall responses in the same host. In contrast, heterologous rechallenge with a pathogen sharing only a CD4(+) T cell epitope results in robust boosting of CD4(+) T cell recall responses. The disparity in CD4(+) and CD8(+) T cell recall responses cannot be attributed to competition for growth factors or APCs, as robust CD4(+) and CD8(+) T cell recall responses can be simultaneously induced following rechallenge with peptide-pulsed dendritic cells. Instead, CD4(+) T cell recall responses are dependent on the duration of the secondary challenge. Increasing the rechallenge dose results in more potent boosting of CD4(+) T cell recall responses and artificially limiting the duration of secondary infection following heterologous rechallenge adversely impacts the magnitude of CD4(+) T cell, but not CD8(+) T cell, recall responses. These findings suggest that rapid pathogen clearance by secondary CTL following homologous rechallenge prevents optimal boosting of CD4(+) T cell responses and therefore have important practical implications in the design of vaccination and boosting strategies aimed at promoting CD4(+) T cell-mediated protection.

Tracking epitope-specific T cells

The tracking of antigen-specific T cells in vivo is a useful approach for the study of the adaptive immune response. This protocol describes how populations of T cells specific for a given peptide-major histocompatibility complex (pMHC) epitope can be tracked based solely on T-cell receptor (TCR) specificity as opposed to other indirect methods based on function. The methodology involves the adoptive transfer of TCR transgenic T cells with defined epitope specificity into histocompatible mice and the subsequent detection of these cells through the use of congenic or clonotypic markers. Alternatively, endogenous epitope-specific T cells can be tracked directly through the use of pMHC tetramers. Using magnetic bead-based enrichment and advanced multiparameter flow cytometry, populations as small as five epitope-specific T cells can be detected from the peripheral lymphoid organs of a mouse. The adoptive transfer procedure can be completed within 3 h, whereas analysis of epitope-specific cells from mice can be completed within 6 h.

Direct stimulation of tlr5+/+ CD11c+ cells is necessary for the adjuvant activity of flagellin

Flagellin is a highly effective adjuvant, but the cellular mechanism underlying this activity remains uncertain. More specifically, no consensus exists as to whether flagellin activates dendritic cells (DC) directly or indirectly. Intramuscular immunization with flagellin-OVA fusion protein resulted in enhanced in vivo T cell clustering in draining lymph nodes and IL-2 production by OVA-specific CD4(+) T cells. Immunization with flagellin-OVA also triggered greater levels of Ag-specific CD4(+) T cell proliferation than immunization with flagellin and OVA as separate proteins. To determine whether flagellin, in the context of a fusion protein with OVA, was acting directly on DC, we used a combination of CD4(+) T cell adoptive transfers and bone marrow chimera mice in which the presence or absence of potential tlr5(+/+) CD11c(+) cells was controlled by injection of diphtheria toxin. The Ag-specific CD4(+) T cell response in mice with CD11c(+) cells from a tlr5(-/-) background and mixed populations of all other hematopoietic cells was dramatically reduced in comparison to mice that had DC from tlr5(-/-) and wild-type backgrounds. Immunization of MyD88(-/-)tlr5(+/+) mice revealed that the enhanced response following immunization with flagellin-OVA is dependent on signaling via the TLR5-MyD88 pathway as well as enhanced Ag uptake and processing resulting from Ag targeting via TLR5. In summary, our data are consistent with the conclusion that direct stimulation of tlr5(+/+) CD11c(+) cells is necessary for the adjuvant activity of a flagellin fusion protein and that this adjuvant effect requires signaling through TLR5.

Self-antigen-specific CD8+ T cell precursor frequency determines the quality of the antitumor immune response

A primary goal of cancer immunotherapy is to improve the naturally occurring, but weak, immune response to tumors. Ineffective responses to cancer vaccines may be caused, in part, by low numbers of self-reactive lymphocytes surviving negative selection. Here, we estimated the frequency of CD8⁺ T cells recognizing a self-antigen to be <0.0001% (∼1 in 1 million CD8⁺ T cells), which is so low as to preclude a strong immune response in some mice. Supplementing this repertoire with naive antigen-specific cells increased vaccine-elicited tumor immunity and autoimmunity, but a threshold was reached whereby the transfer of increased numbers of antigen-specific cells impaired functional benefit, most likely because of intraclonal competition in the irradiated host. We show that cells primed at precursor frequencies below this competitive threshold proliferate more, acquire polyfunctionality, and eradicate tumors more effectively. This work demonstrates the functional relevance of CD8⁺ T cell precursor frequency to tumor immunity and autoimmunity. Transferring optimized numbers of naive tumor-specific T cells, followed by in vivo activation, is a new approach that can be applied to human cancer immunotherapy. Further, precursor frequency as an isolated variable can be exploited to augment efficacy of clinical vaccine strategies designed to activate any antigen-specific CD8⁺ T cells.

Generation and maintenance of memory CD4(+) T Cells

In the course of an immune response to an infectious microbe, pathogen-specific naïve CD4(+) T cells proliferate extensively and differentiate into effector cells. Most of these cells die rapidly, but a small fraction of effector cells persist as memory cells to confer enhanced protection against the same pathogen. Recent advances indicate that strong TCR stimulation during the primary response is essential for the generation of long-lived memory CD4(+) T cells. Memory cells appear to be derived equally from all subsets of effector cells, and memory cells can also acquire additional functional capabilities during the secondary response. Resting memory CD4(+) cells are dependent on signals from contact with IL-7 and IL-15, but not MHC class II, for their survival and intermittent homeostatic proliferation.

Dendritic cell antigen presentation drives simultaneous cytokine production by effector and regulatory T cells in inflamed skin

Effector (Teff) and regulatory (Treg) T cells produce cytokines that balance immunity and immunopathology at sites of infection. It is not known how this balance is achieved. Here, we show that Treg and Teff cells specific for the same foreign peptide:major histocompatibility complex II (pMHCII) ligand accumulated preferentially in a subcutaneous site injected with the relevant antigen plus an adjuvant. Some of the Treg cells in this site were producing IL-10 12 days after injection, whereas a similar fraction of the Teff cells were producing IFN-gamma. Acute ablation of Treg cells increased the fraction of IFN-gamma-producing Teff cells, indicating that Teff function was limited by the Treg cells. Production of cytokines by both populations was driven by pMHCII presentation by local CD11b(hi) dermal dendritic cells. Therefore, balanced production of microbicidal and suppressive cytokines in inflamed skin is achieved by simultaneous dendritic cell antigen presentation to Teff and Treg cells.

Tracking the dynamics of salmonella specific T cell responses

Over the last decade, significant advances have been made in the methodology for studying immune responses in vivo. It is now possible to follow almost every aspect of pathogen-specific immunity using in vivo models that incorporate physiological infectious doses and natural routes of infection. This new ability to study immunity in a relevant physiological context will greatly expand our understanding of the dynamic interplay between host and pathogen. Visualizing the resolution of primary infection and the development of long-term immunological memory should also aid the development of new vaccines and therapeutics for infectious diseases. In this review, we will describe the application of in vivo visualization technology to Salmonella infection, describe our current understanding of Salmonella-specific immunity, and discuss some unanswered questions that remain in this model.

Application of a Systems Biology Approach to Skin Allergy Risk Assessment

We have developed an in silico model of the induction of skin sensitisation, in order to characterise and quantify the contribution of each pathway to the overall biological process. This analysis has been used to guide our research on skin sensitisation and in vitro test development programmes, and provides a theoretical rationale for the interpretation and integration of non-animal predictive data for risk assessment (RA) purposes. The in vivo mouse Local Lymph Node Assay (LLNA) is now in widespread use for the evaluation of skin sensitisation potential and potency. Recent changes in European Union (EU) legislation (i.e. the 7th Amendment to the EU Cosmetics Directive) have made the development of non-animal approaches to provide the data for skin sensitisation RA a key business need. Several in vitro predictive assays have already been developed for the prediction of skin sensitisation. However, these are based on the determination of a small number of pathways within the overall biological process, and our understanding of the relative contribution of these individual pathways to skin sensitisation induction is limited. To address this knowledge gap, a “systems biology” approach has been used to construct a computer-based mathematical model of the induction of skin sensitisation, in collaboration with Entelos, Inc. The biological mechanisms underlying the induction phase of skin sensitisation are represented by nonlinear ordinary differential equations and defined by using information from over 500 published papers. By using the model, we have identified knowledge gaps for future investigative research, and key factors that have a major influence on the induction of skin sensitisation (e.g. TNF-α production in the epidermis). The relative contribution of each of these key pathways has been assessed by determining their contributions to the overall process (e.g. sensitiser-specific T-cell proliferation in the draining lymph node). This information provides a biologically-relevant rationale for the interpretation and potential integration of diverse types of non-animal predictive data. Consequently, the Skin Sensitisation Physiolab® (SSP) platform represents one approach to integration that is likely to prove an invaluable tool for hazard evaluation in a new framework for consumer safety RA.

Increasing the CD4+ T cell precursor frequency leads to competition for IFN-gamma thereby degrading memory cell quantity and quality

The precursor frequency of naive CD4(+) T cells shows an inverse relationship with the number of memory cells generated after exposure to cognate Ag. Using the lymphocytic choriomeningitis virus (LCMV) model, we show here that only when the initial number of naive virus-specific CD4(+) T cell precursors is low (< or =10(4) per spleen) do they give rise to abundant and homogeneous memory cells that are CD62L(low), IL-7R(high), and imbued with an enhanced capacity to produce cytokine, proliferate, and survive over time. Furthermore, memory cells derived from a high naive precursor number show functional deficits upon secondary exposure to virus. The negative effect of higher naive precursor frequency was not attributable to competition for limiting amounts of Ag, because LCMV-naive CD4(+) TCR-transgenic CD4 T cells were recruited into the LCMV-induced response even when their initial number was high. Instead, the T cells appear to compete for direct IFN-gamma signals as they differentiate into memory cells. These results are consistent with a model of T cell development in which the most fit effector T cells that receive sufficient direct IFN-gamma signals are selected to differentiate further into memory cells.

Antigen-specific T-T interactions regulate CD4 T-cell expansion

The regulation of CD4 T-cell numbers during an immune response should take account of the amount of antigen (Ag), the initial frequency of Ag-specific T cells, the mix of naive versus experienced cells, and (ideally) the diversity of the repertoire. Here we describe a novel mechanism of T-cell regulation that potentially deals with all of these parameters. We found that CD4 T cells establish a negative feedback loop by capturing their cognate major histocompatibility class (MHC)/peptide complexes from Ag-presenting cells and presenting them to Ag-experienced CD4 T cells, thereby inhibiting their recruitment into the response while allowing recruitment of naive T cells. The inhibition is Ag specific, begins at day 2 (long before Ag disappearance), and cannot be overcome by providing new Ag-loaded dendritic cells. In this way, CD4 T-cell proliferation is regulated in a functional relationship to the amount of Ag, while allowing naive T cells to generate repertoire variety.

Both CD4+ and CD8+ T cells respond to antigens fused to anthrax lethal toxin

The lethal toxin produced by Bacillus anthracis is a bipartite toxin in which the first protein, protective antigen (PA), transports the second protein, lethal factor, across the host cell membrane. We have previously shown that CD8(+) T-cell epitopes fused to a nontoxic derivative of lethal factor (LFn) are delivered into the host cell cytosol in a PA-dependent manner. Delivery of these antigens targets them to the intracellular major histocompatibility complex (MHC) class I processing and presentation pathway and leads to the stimulation of antigen-specific CD8(+) T cells in vivo. In this report, we describe the generation and characterization of LFn fusion proteins that include not only a CD8(+) T-cell epitope but also a CD4(+) T-cell epitope. We first show that these fusion proteins induce antigen-specific CD4(+) T-cell responses following incubation with dendritic cells in vitro or injection into mice. Stimulation of CD4(+) T cells by LFn fusion proteins does not require PA but is enhanced by PA in vitro. We also show that a single LFn fusion protein and PA can deliver antigen to both the MHC class II and the MHC class I pathways, resulting in the simultaneous induction of antigen-specific CD4(+) T cells and antigen-specific CD8(+) T cells in the same mouse. These results suggest that this toxin delivery system is capable of stimulating protective immune responses where effective immunization requires stimulation of both classes of T cells.

Decoding the dynamics of T cell-dendritic cell interactions in vivo

T lymphocytes receive activation signals during their encounters with antigen-bearing dendritic cells (DCs) in secondary lymphoid organs. With the recent application of two-photon imaging to visualize immune responses as they happen, the dynamics of T cell-DC interactions have been dissected in several mouse models. As we are integrating the results of these new studies, we are learning that the dynamics of T cell-DC interactions are regulated by multiple immunological parameters and, most importantly, that the spatiotemporal characteristics of these cell-cell contacts encode part of the T-cell fate.

Rapid culling of the CD4+ T cell repertoire in the transition from effector to memory

Requirements for CD4+ T cell memory differentiation were analyzed with adoptively transferred SMARTA T cell receptor (TCR) transgenic cells specific for alymphocytic choriomeningitis virus (LCMV) epitope. LCMV-induced effector and memory differentiation of SMARTA cells mimicked the endogenous CD4+ T cell response. In contrast, infection with a recombinant Listeria expressing the LCMV epitope, although resulting initially in massive SMARTA expansion, led to loss of effector function and rapid cell death characterized by high expression of the apoptosis regulator Bim. Defective memory differentiation was seen after stimulation of naive but not memory SMARTA cells, was independent of precursor frequency, and correlated with a lower TCR avidity compared to endogenous responders. In addition, long-lived endogenous CD4+ memory T cells skewed to a higher functional avidity over time. These results support a model in which CD4+ T cell memory differentiation and longevity depend on the strength of the TCR signal during the primary response.

Age-dependent tolerance to an endogenous tumor-associated antigen

Immunologic tolerance to endogenous antigens reduces antitumor responses. Gp70 is an endogenous tumor-associated antigen (TAA) of the BALB/c-derived colon carcinoma CT26. We found that expression of gp70 mRNA is detectable in tissues of mice 8 months of age and older. We showed that expression of gp70 establishes immunologic tolerance and affects antitumor immunity in a similarly age-dependent manner using gp70-deficient mice. We found that tumors grew in all gp70-sufficient mice, while approximately half of gp70-deficient mice controlled tumor growth with endogenous T-cell responses. Protection in gp70-deficient mice correlated with more robust gp70-specific CTL responses, and increased numbers and avidity of responding antigen-specific T cells after vaccination. We conclude that immunosurveillance may decline with age due to increased or de novo peripheral expression of endogenous TAAs.

Initiation of the adaptive immune response to Mycobacterium tuberculosis depends on antigen production in the local lymph node, not the lungs

The onset of the adaptive immune response to Mycobacterium tuberculosis is delayed compared with that of other infections or immunization, and allows the bacterial population in the lungs to expand markedly during the preimmune phase of infection. We used adoptive transfer of M. tuberculosis Ag85B-specific CD4(+) T cells to determine that the delayed adaptive response is caused by a delay in initial activation of CD4(+) T cells, which occurs earliest in the local lung-draining mediastinal lymph node. We also found that initial activation of Ag85B-specific T cells depends on production of antigen by bacteria in the lymph node, despite the presence of 100-fold more bacteria in the lungs. Although dendritic cells have been found to transport M. tuberculosis from the lungs to the local lymph node, airway administration of LPS did not accelerate transport of bacteria to the lymph node and did not accelerate activation of Ag85B-specific T cells. These results indicate that delayed initial activation of CD4(+) T cells in tuberculosis is caused by the presence of the bacteria in a compartment that cannot be mobilized from the lungs to the lymph node, where initial T cell activation occurs.

Increased competition for antigen during priming negatively impacts the generation of memory CD4 T cells

The factors involved in the differentiation of memory CD4 T cells from naïve precursors are poorly understood. We developed a system to examine the effect of increased competition for antigen by CD4 T cells on the generation of memory in response to infection with a recombinant vesicular stomatitis virus. Competition was initially regulated by increasing the precursor frequency of adoptively transferred naïve T cell antigen receptor transgenic CD4 T cells. Despite robust proliferation at high precursor frequencies, memory CD4 T cells did not develop, whereas decreasing the input number of naïve CD4 T cells promoted memory development after infection. The lack of memory development was linked to reduced blastogenesis and poor effector cell induction, but not to initial recruitment or proliferation of antigen-specific CD4 T cells. To prove that availability of antigen alone could regulate memory CD4 T cell development, we used treatment with an mAb specific for the epitope recognized by the transferred CD4 T cells. At high doses, this mAb effectively inhibited the antigen-specific CD4 T cell response. However, at a very low dose of mAb, primary CD4 T cell expansion was unaffected, although memory development was dramatically reduced. Moreover, the induction of effector function was concomitantly inhibited. Thus, competition for antigen during CD4 T cell priming is a major contributing factor to the development of the memory CD4 T cell pool.

Naive CD4(+) T cell frequency varies for different epitopes and predicts repertoire diversity and response magnitude

Cell-mediated immunity stems from the proliferation of naive T lymphocytes expressing T cell antigen receptors (TCRs) specific for foreign peptides bound to host major histocompatibility complex (MHC) molecules. Because of the tremendous diversity of the T cell repertoire, naive T cells specific for any one peptide:MHC complex (pMHC) are extremely rare. Thus, it is not known how many naive T cells of any given pMHC specificity exist in the body or how that number influences the immune response. By using soluble pMHC class II (pMHCII) tetramers and magnetic bead enrichment, we found that three different pMHCII-specific naive CD4(+) T cell populations vary in frequency from 20 to 200 cells per mouse. Moreover, naive population size predicted the size and TCR diversity of the primary CD4(+) T cell response after immunization with relevant peptide. Thus, variation in naive T cell frequencies can explain why some peptides are stronger immunogens than others.