Spatial heterogeneity and peptide availability determine CTL killing efficiency in vivo.

Area of Science:

• Immunology
• Cellular Biology
• Computational Biology

Background:

• Estimating cytotoxic T lymphocyte (CTL) surveying rates is crucial for modeling immune responses to intracellular pathogens.
• In vivo cytotoxicity assays using peptide-pulsed splenocytes are common but occur in a heterogeneous splenic environment.
• Questions remain about cell-type specific lysis susceptibility, spatial distributions, and peptide-MHC levels.

Purpose of the Study:

• To investigate factors influencing CTL killing efficiency in vivo.
• To determine if different splenocyte populations have intrinsic differences in susceptibility to CTL lysis.
• To dissect the impact of spatial distribution, effector/target interactions, and peptide-MHC levels on CTL-mediated killing.

Main Methods:

• Utilized an in vivo splenocyte killing assay with influenza virus-specific CTLs.
• Employed quantitative imaging, in vitro killing assays, and mathematical modeling.
• Analyzed CTL-target conjugate formation, lysis probability, and killing kinetics.

Main Results:

• At the population level, T cell targets were killed more rapidly than B cell targets in vivo.
• In vivo differences were attributed to target cell migration and CTL distribution, not intrinsic lysis susceptibility.
• In vitro, peptide dose affected CTL-target conjugation but not lysis probability; T cell targets lysed slower than B cells.
• Incomplete in vivo killing at low peptide doses may result from heterogeneous peptide uptake and peptide-MHC complex dissociation.

Conclusions:

• Splenic heterogeneity in cell migration and CTL distribution explains observed in vivo killing rate differences.
• Intrinsic susceptibility to CTL lysis is similar between T and B cells.
• Modeling reveals distinct stages of CTL-target interaction, with peptide dose influencing initial engagement rather than killing efficiency.
• Dissecting population-averaged parameters is key to understanding spatial and cellular heterogeneity in immune response models.