Modelling the impact of antigen kinetics on T‐cell activation and response

Partitioning regulatory mechanisms of within-host malaria dynamics using the effective propagation number

Immune clearance and resource limitation (via red blood cell depletion) shape the peaks and troughs of malaria parasitemia, which in turn affect disease severity and transmission. Quantitatively partitioning the relative roles of these effects through time is challenging. Using data from rodent malaria, we estimated the effective propagation number, which reflects the relative importance of contrasting within-host control mechanisms through time and is sensitive to the inoculating parasite dose. Our analysis showed that the capacity of innate responses to restrict initial parasite growth saturates with parasite dose and that experimentally enhanced innate immunity can affect parasite density indirectly via resource depletion. Such a statistical approach offers a tool to improve targeting of drugs or vaccines for human therapy by revealing the dynamics and interactions of within-host regulatory mechanisms.

Rapid assessment of recognition efficiency and functional capacity of antigen-specific T-cell responses

It is increasingly recognized that cells within an antigen-specific CD8 T-cell population may be diverse in recognition efficiency for target, which may significantly affect the overall efficacy of the response in clinical settings such as viral infections and cancer. CD8 T cells with seemingly identical antigen specificity, particularly those elicited by cancer vaccines, may be heterogeneous for sensitivity and recognition efficiency for the cognate peptide and functional state in vivo. Analysis of individual T-cell clones derived from an antigen-specific T-cell population would provide an accurate assessment of the overall response; however, this is time- and labor-intensive, preventing rapid and routine assessment of patient samples from clinical trials. By stimulating antigen-specific T cells that otherwise appear homogeneous on tetramer staining with graded amounts of cognate peptides, the authors show that individual cells downmodulate surface T-cell receptors (TCR) and thus lose tetramer reactivity with variable dynamics within the T-cell population. The dynamics of TCR downregulation represent an accurate assessment of an individual cell's antigen sensitivity, recognition efficiency, and relative functional state within an antigen-specific population and have direct correlation to killing capacity by chromium release as well as degranulation by CD107 mobilization. Furthermore, despite correlation of average T-cell function by all three techniques, TCR downregulation uncovered heterogeneity in T-cell responses after vaccination among patient samples directly ex vivo. When examined using this novel technique, antigen-specific T cells elicited by vaccination with heteroclitic peptides exhibited significantly different recognition efficiencies for the heteroclitic versus native peptides, translating into differences in functional responses. With advancing cancer vaccine trials, the capacity to detect and functionally characterize antigen-specific T-cell responses in detail is critical. Techniques, as presented here, that rapidly assess the overall antigen sensitivity, recognition efficiency, and functional status of patients' T-cell responses will guide future vaccine trials and immunotherapies.

Distinct temporal programming of naive CD4+ T cells for cell division versus TCR-dependent death susceptibility by antigen-presenting macrophages

Naive T cells become programmed for clonal expansion and contraction during the early hours of antigenic signaling. Recent studies support an 'autopilot' model, wherein the commitment to proliferate and the magnitude of the proliferative response are simultaneously determined during a single, brief period of antigen exposure. Here, we have examined whether the proliferation of naive CD4+ T cells must occur on 'autopilot', or whether extended periods of antigenic signaling can impact primary proliferative responses to antigen-presenting macrophages (macrophage APC). We found that a single exposure to antigen (18 h) simultaneously committed T cells to (1) up-regulate surface TCR above the level expressed on naive T cells, (2) undergo minimal cell division, and (3) acquire susceptibility to TCR-dependent activation-induced cell death. However, continued antigenic signaling between 18 and 72 h was required to amplify the number of daughter cells derived from the already committed T cells. Thus, a discrete commitment time was followed by a 'tuning' period, where extended antigenic signaling determined the volume of the proliferative response. We conclude that T cell commitment to full clonal expansion versus TCR-dependent death susceptibility represent two separate programming events whose timing can be segregated by macrophage APC.

The role of models in understanding CD8+ T-cell memory

Immunological memory - the ability to 'remember' previously encountered pathogens and respond faster on re-exposure - is a central feature of the immune response of vertebrates. We outline how mathematical models have contributed to our understanding of CD8(+) T-cell memory. Together with experimental data, models have helped to quantitatively describe and to further our understanding of both the generation of memory after infection with a pathogen and the maintenance of this memory throughout the life of an individual.