Estimating the Precursor Frequency of Naive Antigen-specific CD8 T Cells

TLDR

The limited lymphocyte pool forces precursor frequencies of antigen‑specific naive CD8 T cells to be below direct detection limits, and adoptive transfer of transgenic cells into syngeneic recipients with antigen stimulation yields dose‑dependent activation and expansion of both donor and endogenous cells. The study estimates precursor frequency by titrating a known quantity of antigen‑specific cells into naive recipients. The method equates precursor frequency to the transferred cell count when transgenic and endogenous responses are of equal magnitude. The method estimates a precursor frequency of 1 in 2×10⁵, corresponding to 100–200 GP33‑specific naive CD8 T cells in an uninfected mouse, which expand >14‑fold in one week to ~10⁷ cells and generate a memory pool of ~5×10⁵ cells, a >1,000‑fold increase after LCMV infection.

Abstract

The constraint of fitting a diverse repertoire of antigen specificities in a limited total population of lymphocytes results in the frequency of naive cells specific for any given antigen (defined as the precursor frequency) being below the limit of detection by direct measurement. We have estimated this precursor frequency by titrating a known quantity of antigen-specific cells into naive recipients. Adoptive transfer of naive antigen-specific T cell receptor transgenic cells into syngeneic nontransgenic recipients, followed by stimulation with specific antigen, results in activation and expansion of both donor and endogenous antigen-specific cells in a dose-dependent manner. The precursor frequency is equal to the number of transferred cells when the transgenic and endogenous responses are of equal magnitude. Using this method we have estimated the precursor frequency of naive CD8 T cells specific for the H-2Db–restricted GP33–41 epitope of LCMV to be 1 in 2 × 105. Thus, in an uninfected mouse containing ∼2-4 × 107 naive CD8 T cells we estimate there to be 100–200 epitope-specific cells. After LCMV infection these 100–200 GP33-specific naive CD8 T cells divide >14 times in 1 wk to reach a total of ∼107 cells. Approximately 5% of these activated GP33-specific effector CD8 T cells survive to generate a memory pool consisting of ∼5 × 105 cells. Thus, an acute LCMV infection results in a >1,000-fold increase in precursor frequency of DbGP33-specific CD8 T cells from 2 × 102 naive cells in uninfected mice to 5 × 105 memory cells in immunized mice.