Protection against immunopathological consequences of a viral infection by activated but not resting cytotoxic T cells: T cell memory without "memory T cells"?

Abstract

Immunological memory is a key characteristic of specific immune responses. Persistence of increased levels of precursor T cells is antigen-independent and is often used as an indicator of T cell memory. This study documents that, depending on the chosen readout, cytotoxic T lymphocyte (CTL) memory against lymphocytic choriomeningitis virus (LCMV) appears long- or short-lived in the absence of persisting antigen. To study T cell memory in the absence of persisting antigen, either short-lived antigens were used for immunization or adoptive transfer methods were used to eliminate possibly persisting antigen. These experiments revealed that increased specific precursor frequencies and CTL-mediated protection against an i.v. infection with LCMV were long-lived. In contrast, CTL-mediated protection against a peripheral infection of the skin with LCMV, or of the ovary with recombinant vaccinia virus, was short-lived. These results show that maintenance of increased specific CTL precursor frequencies and central T cell memory in lymphoid tissue (where preexisting neutralizing antibodies usually provide protection anyway) is long-lived and antigen-independent. In contrast, in protection against peripheral viral infections, where the relative kinetics of virus growth and virus elimination by T cells are of key importance, T cell memory is short-lived in the absence of antigen. This indicates that peripheral T cell memory in antibody-inaccessible tissues is mediated by antigen-activated effector T cells and apparently not by specialized memory T cells.

Figure 1

Induction of long-lived increased CTLp frequencies with GP2.9 cells. Mice were immunized i.p. with irradiated (2500 cGy) GP2.9 cells (3 × 10⁶ cells) (solid triangles) or left untreated (open squares). At the indicated time points, spleen cells were restimulated for 5 days in vitro and tested in a conventional ⁵¹Cr release assay on EL-4 cells pulsed with the relevant peptide (Upper). Alternatively, mice were challenged i.v. with LCMV (strain WE, 200 pfu), and virus titers in spleens were determined 5 days later (Lower). The horizontal line indicates the detection limit. The number of points below the line indicates the number of mice with no detectable virus. One representative experiment of two is shown.

Figure 2

Antigen persisting as immune complexes on FDCs are not necessary to maintain CTL memory. Mice deficient for IgM expression (−/−, solid triangles) that cannot produce antibody and control mice (+/+, open triangles) were immunized with 10 μg of recombinant LCMV-GP protein (Upper) or 10 μg of recombinant VSV-N protein (Lower). Eight to 12 weeks later, spleen cells were restimulated in vitro with the relevant peptides and tested 5 days later in a conventional ⁵¹Cr release assay on peptide-pulsed EL-4 cells. Alternatively, LCMV-GP-primed mice were challenged with LCMV (200 pfu), and virus titers were determined in the spleen 5 days later. VSV-N primed mice were challenged i.p. with recombinant vaccinia virus expressing VSV-N (5 × 10⁶ pfu), and virus titers were determined 5 days later in ovaries. The horizontal line indicates the detection limit. One representative experiment of three is shown.

Figure 3

Antigen dependence of protective CTL memory varies with the readout: i.v. challenge infection with different virus doses. Mice were immunized with irradiated (25 Gy) GP2.9 cells (3 × 10⁶ or 10⁵ cells) and frequencies of LCMV-GP-specific CTLp were determined 10 (solid triangles) or 60 days (open triangles) later. Alternatively, mice were challenged i.v. with 200 pfu of LCMV WE, and virus titers were assessed 5 days later, or they were challenged with 10⁴ pfu of LCMV WE and virus titers were assessed 3 days later. Additional groups of mice were challenged with 10⁶ pfu of LCMV Cl-13, and virus titers were assessed in the blood 10 days later. The horizontal line indicates the detection limit. One representative experiment of two is shown.