title: Spatial heterogeneity and peptide availability determine CTL killing efficiency in vivo.
abstract: The rate at which a cytotoxic T lymphocyte (CTL) can survey for infected cells is a key ingredient of models of vertebrate immune responses to intracellular pathogens. Estimates have been obtained using in vivo cytotoxicity assays in which peptide-pulsed splenocytes are killed by CTL in the spleens of immunised mice. However the spleen is a heterogeneous environment and splenocytes comprise multiple cell types. Are some cell types intrinsically more susceptible to lysis than others? Quantitatively, what impacts are made by the spatial distribution of targets and effectors, and the level of peptide-MHC on the target cell surface? To address these questions we revisited the splenocyte killing assay, using CTL specific for an epitope of influenza virus. We found that at the cell population level T cell targets were killed more rapidly than B cells. Using modeling, quantitative imaging and in vitro killing assays we conclude that this difference in vivo likely reflects different migratory patterns of targets within the spleen and a heterogeneous distribution of CTL, with no detectable difference in the intrinsic susceptibilities of the two populations to lysis. Modeling of the stages involved in the detection and killing of peptide-pulsed targets in vitro revealed that peptide dose influenced the ability of CTL to form conjugates with targets but had no detectable effect on the probability that conjugation resulted in lysis, and that T cell targets took longer to lyse than B cells. We also infer that incomplete killing in vivo of cells pulsed with low doses of peptide may be due to a combination of heterogeneity in peptide uptake and the dissociation, but not internalisation, of peptide-MHC complexes. Our analyses demonstrate how population-averaged parameters in models of immune responses can be dissected to account for both spatial and cellular heterogeneity.
authors.affiliation: Immune Cell Biology, MRC National Institute for Medical Research, Mill Hill, London, United Kingom.
authors.forename: Thea
authors.initials: T
authors.lastname: Hogan
authors.affiliation: Department of Systems and Computational Biology, Albert Einstein College of Medicine, New York, New York, United States of America.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Kadolsky
authors.affiliation: Immune Cell Biology, MRC National Institute for Medical Research, Mill Hill, London, United Kingom.
authors.forename: Sim
authors.initials: S
authors.lastname: Tung
authors.affiliation: Institute of Immunity and Transplantation, Division of Infection and Immunity, UCL, Royal Free Hospital, London, United Kingdom.
authors.forename: Benedict
authors.initials: B
authors.lastname: Seddon
authors.affiliation: Department of Systems and Computational Biology, Albert Einstein College of Medicine, New York, New York, United States of America; Department of Microbiology and Immunology, Albert Einstein College of Medicine, New York, New York, United States of America; Institute of Infection, Immunity & Inflammation, College of Medical, Veterinary & Life Sciences, University of Glasgow, Glasgow, United Kingdom.
authors.forename: Andrew
authors.initials: A
authors.lastname: Yates
mesh_terms: D000818:Animals; D001402:B-Lymphocytes / Q000276:immunology; D016923:Cell Death / Q000276:immunology; D004195:Disease Models, Animal; D006801:Humans; D007251:Influenza, Human / Q000276:immunology; D051379:Mice; D008822:Mice, Transgenic; D018448:Models, Immunological; D010455:Peptides / Q000008:administration & dosage / Q000276:immunology*; D013154:Spleen / Q000166:cytology; D013602:T-Lymphocytes, Cytotoxic / Q000276:immunology*
chemical_list: D010455:Peptides