Proliferating CD4+ T cells undergo immediate growth arrest upon cessation of TCR signaling in vivo

The Role of Antigen-Competitive Dynamics in Regulating the Immune Response

The clonal expansion of T cells during an infection is tightly regulated to ensure an appropriate immune response against invading pathogens. Although experiments have mapped the trajectory from expansion to contraction, the interplay between mechanisms that control this response is not fully understood. Based on experimental data, we propose a model in which the dynamics of CD4+ T cell expansion is controlled through the interactions between T cells and antigen-presenting cells, where T cell stimulation is proportional to antigen availability, and antigen availability is regulated through downregulation of antigen by T cells. This antigen-dependent-feedback mechanism operates alongside an intrinsic reduction in cell proliferation rate that may also be responsible for slowing expansion. Our model can successfully predict T cell recruitment rates into division, expansion, and clonal burst size per cell when initial precursors are varied or when T cells are introduced late into an ongoing immune response. Importantly, the findings demonstrate that a feedback mechanism between T cells and antigen-presenting cells, along with a reduction in cell proliferation rate, can explain the ability of the immune system to adapt its response to variations in initial conditions or changes that occur later in the response, ensuring a robust yet controlled line of defence against pathogens.

Selective pre-priming of HA-specific CD4 T cells restores immunological reactivity to HA on heterosubtypic influenza infection

A hallmark of the immune response to influenza is repeated encounters with proteins containing both genetically conserved and variable components. Therefore, the B and T cell repertoire is continually being remodeled, with competition between memory and naïve lymphocytes. Our previous work using a mouse model of secondary heterosubtypic influenza infection has shown that this competition results in a focusing of CD4 T cell response specificity towards internal virion proteins with a selective decrease in CD4 T cell reactivity to the novel HA epitopes. Strikingly, this shift in CD4 T cell specificity was associated with a diminished anti-HA antibody response. Here, we sought to determine whether the loss in HA-specific reactivity that occurs as a consequence of immunological memory could be reversed by selectively priming HA-specific CD4 T cells prior to secondary infection. Using a peptide-based priming strategy, we found that selective expansion of the anti-HA CD4 T cell memory repertoire enhanced HA-specific antibody production upon heterosubtypic infection. These results suggest that the potentially deleterious consequences of repeated exposure to conserved influenza internal virion proteins could be reversed by vaccination strategies that selectively arm the HA-specific CD4 T cell compartment. This could be a potentially useful pre-pandemic vaccination strategy to promote accelerated neutralizing antibody production on challenge with a pandemic influenza strain that contains few conserved HA epitopes.

Human regulatory T cells control TCR signaling and susceptibility to suppression in CD4+ T cells

Human CD4(+)CD25(hi)FOXP3(+) regulatory T cells maintain immunologic tolerance and prevent autoimmune and inflammatory immune responses. Regulatory T cells undergo a similar activation cycle as conventional CD4(+) T cells upon antigen stimulation. Here, we demonstrate that T cell receptors and costimulation are required to activate the regulatory T cell suppressive function. Regulatory T cells suppressed the T cell receptor signaling in effector T cells in a time-dependent manner that corresponded with inhibition of cytokine production and proliferation. Modulation of the activation level and thereby the suppressive capacity of regulatory T cells imposed distinct T cell receptor signaling signatures and hyporesponsiveness in suppressed and proliferating effector T cells and established a threshold for effector T cell proliferation. The immune suppression of effector T cells was completely reversible upon removal of regulatory T cells. However, the strength of prior immune suppression by regulatory T cells and corresponding T cell receptor signaling in effector T cells determined the susceptibility to suppression upon later reexposure to regulatory T cells. These findings demonstrate how the strength of the regulatory T cell suppressive function determines intracellular signaling, immune responsiveness, and the later susceptibility of effector T cells to immune suppression and contribute to unveiling the complex interactions between regulatory T cells and effector T cells.

The Timing of T Cell Priming and Cycling

The proliferation of specific lymphocytes is the central tenet of the clonal selection paradigm. Antigen recognition by T cells triggers a series of events that produces expanded clones of differentiated effector cells. TCR signaling events are detectable within seconds and minutes and are likely to continue for hours and days in vivo. Here, I review the work done on the importance of TCR signals in the later part of the expansion phase of the primary T cell response, primarily regarding the regulation of the cell cycle in CD4(+) and CD8(+) cells. The results suggest a degree of programing by early signals for effector differentiation, particularly in the CD8(+) T cell compartment, with optimal expansion supported by persistent antigen presentation later on. Differences to CD4(+) T cell expansion and new avenues toward a molecular understanding of cell cycle regulation in lymphocytes are discussed.

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 cell differentiation, function, and roles in disease

Follicular helper T (Tfh) cells are specialized providers of T cell help to B cells, and are essential for germinal center formation, affinity maturation, and the development of most high-affinity antibodies and memory B cells. Tfh cell differentiation is a multistage, multifactorial process involving B cell lymphoma 6 (Bcl6) and other transcription factors. This article reviews understanding of Tfh cell biology, including their differentiation, migration, transcriptional regulation, and B cell help functions. Tfh cells are critical components of many protective immune responses against pathogens. As such, there is strong interest in harnessing Tfh cells to improve vaccination strategies. Tfh cells also have roles in a range of other diseases, particularly autoimmune diseases. Overall, there have been dramatic advances in this young field, but there is much to be learned about Tfh cell biology in the interest of applying that knowledge to biomedical needs.

Rapid proliferation of activated lymph node CD4(+) T cells is achieved by greatly curtailing the duration of gap phases in cell cycle progression

Peripheral T cells are in G0 phase and do not proliferate. When they encounter an antigen, they enter the cell cycle and proliferate in order to initiate an active immune response. Here, we have determined the first two cell cycle times of a leading population of CD4⁺ T cells stimulated by PMA plus ionomycin in vitro. The first cell cycle began around 10 h after stimulation and took approximately 16 h. Surprisingly, the second cell cycle was extremely rapid and required only 6 h. T cells might have a unique regulatory mechanism to compensate for the shortage of the gap phases in cell cycle progression. This unique feature might be a basis for a quick immune response against pathogens, as it maximizes the rate of proliferation.

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.

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.

Differential kinetics of antigen dependency of CD4+ and CD8+ T cells

Ag recognition via the TCR is necessary for the expansion of specific T cells that then contribute to adaptive immunity as effector and memory cells. Because CD4+ and CD8+ T cells differ in terms of their priming APCs and MHC ligands we compared their requirements of Ag persistence during their expansion phase side by side. Proliferation and effector differentiation of TCR transgenic and polyclonal mouse T cells were thus analyzed after transient and continuous TCR signals. Following equally strong stimulation, CD4+ T cell proliferation depended on prolonged Ag presence, whereas CD8+ T cells were able to divide and differentiate into effector cells despite discontinued Ag presentation. CD4+ T cell proliferation was neither affected by Th lineage or memory differentiation nor blocked by coinhibitory signals or missing inflammatory stimuli. Continued CD8+ T cell proliferation was truly independent of self-peptide/MHC-derived signals. The subset divergence was also illustrated by surprisingly broad transcriptional differences supporting a stronger propensity of CD8+ T cells to programmed expansion. These T cell data indicate an intrinsic difference between CD4+ and CD8+ T cells regarding the processing of TCR signals for proliferation. We also found that the presentation of a MHC class II-restricted peptide is more efficiently prolonged by dendritic cell activation in vivo than a class I bound one. In summary, our data demonstrate that CD4+ T cells require continuous stimulation for clonal expansion, whereas CD8+ T cells can divide following a much shorter TCR signal.

Soluble antigen traffics rapidly and selectively from the corneal surface to the eye draining lymph node and activates T cells when codelivered with CpG oligonucleotides

The transport of antigen to the secondary lymphoid tissue is a central component in the initiation of the adaptive immune response. The mechanism of antigen delivery to the DLN from the avascular cornea has not been fully explored. Previous studies in the mouse have shown that cell-associated corneal antigen is delivered within 6 h to the eye draining SM DLN via DCs and macrophages. In this study, we used a system in which antigen and the processed p-MHCII complexes derived from the antigen could be tracked in vivo. We report that soluble antigen applied to an abraded cornea in the mouse is transported rapidly (within 30 min) to the SM DLN, where a proportion is taken up by resident DCs and presented as p-MHCII complexes, while the larger part is cleared by 8 h. At a later time, a second wave of antigen transport in migratory DCs enters the DLN and participates in further continued antigen presentation. With the use of an antigen-specific TCR transgenic mouse system, we demonstrate that T cell activation does not occur during the early stages of soluble antigen delivery to LN, even though p-MHCII complexes are generated. Antigen-specific T cell activation occurs in the later, presumed cell-associated phase but requires codelivery of a "danger" signal, such as the TLR ligand CpG. We suggest that the early delivery of soluble antigen is more likely to induce T cell nonresponsiveness (anergy) unless presented in the context of an innate-immune cell activation (danger) signal.

Quantifying T lymphocyte turnover

Peripheral T cell populations are maintained by production of naive T cells in the thymus, clonal expansion of activated cells, cellular self-renewal (or homeostatic proliferation), and density dependent cell life spans. A variety of experimental techniques have been employed to quantify the relative contributions of these processes. In modern studies lymphocytes are typically labeled with 5-bromo-2'-deoxyuridine (BrdU), deuterium, or the fluorescent dye carboxy-fluorescein diacetate succinimidyl ester (CFSE), their division history has been studied by monitoring telomere shortening and the dilution of T cell receptor excision circles (TRECs) or the dye CFSE, and clonal expansion has been documented by recording changes in the population densities of antigen specific cells. Proper interpretation of such data in terms of the underlying rates of T cell production, division, and death has proven to be notoriously difficult and involves mathematical modeling. We review the various models that have been developed for each of these techniques, discuss which models seem most appropriate for what type of data, reveal open problems that require better models, and pinpoint how the assumptions underlying a mathematical model may influence the interpretation of data. Elaborating various successful cases where modeling has delivered new insights in T cell population dynamics, this review provides quantitative estimates of several processes involved in the maintenance of naive and memory, CD4(+) and CD8(+) T cell pools in mice and men.

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.

Tracking antigen-specific CD4+ T cells throughout the course of chronic Leishmania major infection in resistant mice

Primary Leishmania major infection typically produces cutaneous lesions that not only heal but also harbor persistent parasites. While the opposing roles of CD4(+) T-cell-derived IFN-γ and IL-10 in promoting parasite killing and persistence have been well established, how these responses develop from naïve precursors has not been directly monitored throughout the course of infection. We used peptide:Major Histocompatibility Complex class II (pMHCII) tetramers to investigate the endogenous, parasite-specific primary CD4(+) T-cell response to L. major in mice resistant to infection. Maximal frequencies of IFN-γ(+) CD4(+) T cells were observed in the spleen and infected ears within a month after infection and were maintained into the chronic phase. In contrast, peak frequencies of IL-10(+) CD4(+) T cells emerged within 2 weeks of infection, persisted into the chronic phase, and accumulated in the infected ears but not the spleen, via a process that depended on local antigen presentation. T helper type-1 (Th1) cells, not Foxp3(+) regulatory T cells, were the chief producers of IL-10 and were not exhausted. Therefore, tracking antigen-specific CD4(+) T cells revealed that IL-10 production by Th1 cells is not due to persistent T-cell antigen receptor stimulation, but rather driven by early antigen encounter at the site of infection.

Arthritogenic self-reactive CD4+ T cells acquire an FR4hiCD73hi anergic state in the presence of Foxp3+ regulatory T cells

Rheumatoid arthritis develops in association with a defect in peripheral CD4(+) T cell homeostasis. T cell lymphopenia has also been shown to be a barrier to CD4(+) T cell clonal anergy induction. We therefore explored the relationship between clonal anergy induction and the avoidance of autoimmune arthritis by tracking the fate of glucose-6-phosphate isomerase (GPI)-reactive CD4(+) T cells in the setting of selective T cell lymphopenia. CD4(+) T cell recognition of self-GPI peptide/MHC class II complexes in normal murine hosts did not lead to arthritis and instead caused those T cells to develop a Folate receptor 4(hi)CD73(hi) anergic phenotype. In contrast, hosts selectively depleted of polyclonal Foxp3(+)CD4(+) regulatory T cells could not make GPI-specific CD4(+) T cells anergic and failed to control arthritis. This suggests that autoimmune arthritis develops in the setting of lymphopenia when Foxp3(+)CD4(+) regulatory T cells are insufficient to functionally inactivate all autoreactive CD4(+) T cells that encounter self-Ag.

A TCR transgenic mouse reactive with multiple systemic dimorphic fungi

Dimorphic fungi collectively account for 5-10 million new infections annually worldwide. Ongoing efforts seek to clarify mechanisms of cellular resistance to these agents and develop vaccines. A major limitation in studying the development of protective T cells in this group of organisms is the lack of tools to detect, enumerate, and characterize fungus-specific T cells during vaccination and infection. We generated a TCR transgenic mouse (Bd 1807) whose CD4(+) T cells respond to a native epitope in Blastomyces dermatitidis and also in Histoplasma capsulatum. In this study, we characterize the mouse, reveal its applications, and extend our analysis showing that 1807 cells also respond to the related dimorphic fungi Coccidioides posadasii and Paracoccidioides lutzii. On adoptive transfer into vaccinated wild-type mice, 1807 cells become activated, proliferate, and expand in the draining lymph nodes, and they differentiate into T1 effectors after trafficking to the lung upon lethal experimental challenge. Bd 1807 cells confer vaccine-induced resistance against B. dermatitidis, H. capsulatum, and C. posadasii. Transfer of naive 1807 cells at serial intervals postvaccination uncovered the prolonged duration of fungal Ag presentation. Using 1807 cells, we also found that the administration of vaccine only once induced a maximal pool of effector/memory CD4(+) cells and protective immunity by 4 wk after vaccination. The autologous adoptive transfer system described in this study reveals novel features of antifungal immunity and offers a powerful approach to study the differentiation of Ag-specific T cells responsive to multiple dimorphic fungi and the development of CD4(+) T cell memory needed to protect against fungal infection.

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.

The early generation of a heterogeneous CD4+ T cell response to Leishmania major

CD4(+) T cells are an essential component of both the primary and secondary immune response against the intracellular protozoan parasite Leishmania major. Our laboratory has previously shown that CD62L(high) IL-7R(high) central memory T (T(CM)) cells mediate protective immunity following secondary challenge. To determine when T(CM) cells develop, we examined the phenotype of Leishmania-specific CD4(+) T cells in the first 2 wk following infection. As expected, we identified a population of CD4(+) T cells present in the draining lymph node with the characteristics of effector T cells. However, in addition, a second population phenotypically resembling T(CM) cells emerged coincident with the effector population. These T cells, expressing CD62L, CCR7, and IL-7R, failed to produce IFN-gamma, but had the capacity to give rise to IFN-gamma-producing effector cells. Our studies also demonstrated that the degree of proliferation and the timing of lymph node entry impact T(CM) cell development. The early generation of T(CM) cells following L. major infection indicates that T(CM) cells may not only control secondary infections, but may also contribute to the control of the primary infection.

Abortive activation of CD4 T cell responses during competitive priming in vivo

Immunodominance refers to the highly selective peptide reactivity of T cells during an immune response. In this study, we tested the hypothesis that persistence of peptide:class II complexes is one key parameter that selects the final specificity of CD4 T cells. We found that low-stability peptide:class II complexes support the initial priming and expansion of CD4 T cells, but the expansion becomes strikingly aborted in the presence of competitive T cell responses to unrelated peptides. Our experiments revealed that for inhibition to occur, the competitive responses must be initiated by the same antigen presenting cell, and it is not because of competition for MHC binding. These studies not only provide an insight into the events that regulate competitive CD4 T cell priming in vivo, but also provide a previously undescribed conceptual framework to understand the parameters that select the final specificity of the T cell repertoire during pathogen or vaccine-induced immune responses.

Understanding the focused CD4 T cell response to antigen and pathogenic organisms

Immunodominance is a term that reflects the final, very limited peptide specificity of T cells that are elicited during an immune response. Recent experiments in our laboratory compel us to propose a new paradigm for the control of immunodominance in CD4 T cell responses, stating that immunodominance is peptide-intrinsic and is dictated by the off-rate of peptides from MHC class II molecules. Our studies have revealed that persistence of peptide:class II complexes both predicts and controls CD4 T cell immunodominance and that this parameter can be rationally manipulated to either promote or eliminate immune responses. Mechanistically, we have determined that DM editing in APC is a key event that is influenced by the kinetic stability of class II:peptide complexes and that differential persistence of complexes also impacts the expansion phase of the immune response. These studies have important implications for rational vaccine design and for understanding the immunological mechanisms that limit the specificity of CD4 T cell responses.

Casitas B-lineage lymphoma b inhibits antigen recognition and slows cell cycle progression at late times during CD4+ T cell clonal expansion

Optimal clonal expansion of CD4(+) T cells during the primary response to Ag requires prolonged TCR recognition of peptide Ag/MHC complexes. In this study, we investigated the capacity of Casitas B-lineage lymphoma b (Cbl-b) to counter-regulate late TCR signals necessary for continued cell division in vivo. During the first 24 h of a primary response to Ag, Cblb(-/-) 5C.C7 CD4(+) T cells demonstrated no alteration in CD69, CD25, and CD71 up-regulation or cell growth as compared with wild-type cells. Nevertheless, beyond 24 h, both the expression of CD71 and the rate of cell division were increased in the genetic absence of Cbl-b, leading to an augmented clonal expansion. This deregulation of late T cell proliferation in the absence of Cbl-b resulted in part from an inability of Cblb(-/-) T cells to desensitize Akt, PLCgamma-1, and ERK phosphorylation events downstream of the TCR/CD3 complex, in addition to their failure to undergo a growth arrest in the absence of Ag. These observations now suggest a novel role for Cbl-b in triggering the exit from cell cycle at the end of a CD4(+) T cell clonal expansion.

Estimating in vivo death rates of targets due to CD8 T-cell-mediated killing

Despite recent advances in immunology, several key parameters determining virus dynamics in infected hosts remain largely unknown. For example, the rate at which specific effector and memory CD8 T cells clear virus-infected cells in vivo is hardly known for any viral infection. We propose a framework to quantify T-cell-mediated killing of infected or peptide-pulsed target cells using the widely used in vivo cytotoxicity assay. We have reanalyzed recently published data on killing of peptide-pulsed splenocytes by cytotoxic T lymphocytes and memory CD8 T cells specific to NP396 and GP276 epitopes of lymphocytic choriomeningitis virus (LCMV) in the mouse spleen. Because there are so many effector CD8 T cells in spleens of mice at the peak of the immune response, NP396- and GP276-pulsed targets are estimated to have very short half-lives of 2 and 14 min, respectively. After the effector numbers have diminished, i.e., in LCMV-immune mice, the half-lives become 48 min and 2.8 h for NP396- and GP276-expressing targets, respectively. Analysis of several alternative models demonstrates that the estimates of half-life times of peptide-pulsed targets are not affected when changes are made in the model assumptions. Our report provides a unifying framework to compare killing efficacies of CD8 T-cell responses specific to different viral and bacterial infections in vivo, which may be used to compare efficacies of various cytotoxic-T-lymphocyte-based vaccines.