Homeostatic expansion and phenotypic conversion of naïve T cells in response to self peptide/MHC ligands

Abstract

Recent data suggest that survival of resting, naïve T cells requires an interaction with self MHC molecules. From analysis of the class I MHC-restricted T cell receptor transgenic strain OT-I, we report a different response. Rather than merely surviving, these T cells proliferated slowly after transfer into T-depleted syngeneic hosts. This expansion required both T cell "space" and expression of normal levels of self class I MHC molecules. Furthermore, we demonstrate that during homeostatic expansion in a suitable environment, naïve phenotype (CD44(low)) OT-I T cells converted to memory phenotype (CD44(med/high)), despite the absence of foreign antigenic stimulation. On the other hand, cells undergoing homeostatic expansion did not acquire cytolytic effector function. The significance of these data for reactivity of T cells with self peptide/MHC ligands and the implications for normal and abnormal T cell homeostasis are discussed.

Figure 1

Proliferation of OT-I T cells in irradiated syngeneic hosts. OT-I CD8⁺ T cells were purified from Thy1.1 donor mice (OT-I.PL), CFSE labeled and transferred into B6 (A), irradiated B6 (B and D), or irradiated TAP^0/0 (C) hosts. (D) The mice also were immunized with OVAp. Five days after transfer, lymph nodes were recovered, and the CFSE expression on donor CD8⁺ cells was determined. Data are shown for one of the two animals in each group.

Figure 2

The lack of CD8 expansion in irradiated TAP^0/0 recipients is not caused by rejection of transferred T cells. CD8⁺ T cells from OT-I.PL animals and CD4⁺ cells from B6.PL animals were cotransferred into irradiated TAP^0/0 and B6 recipients. One, 5, or 11 days after transfer, lymph nodes and spleens were harvested from three animals per group. The total cell numbers of donor CD4 and CD8 T cells and the CFSE expression of these populations was determined. Total numbers of donor OT-I.PL CD8⁺ (A) and B6.PL CD4⁺ (B) cells are given for irradiated TAP^0/0 (blue symbols) and B6 (red symbols) recipients. The average donor cell number (with SD shown as error bars) for the lymph node population is shown. (C) The CFSE expression by the OT-I CD8+ cells in three TAP^0/0 (blue lines) and three B6 (red lines) recipients at day 5 after transfer.

Figure 3

OT-I T cell proliferation and CD44 up-regulation occur after transfer into T- and T/B- deficient, nonirradiated hosts. OT-I.PL CD8⁺ T cells were transferred into unmanipulated B6 (A and E), B6 RAG^0/0 (B and F), or thymectomized, T cell-depleted B6 (C and G) recipients for 9 days. Expression levels of CFSE (A-C) and staining for CD44 (E-G) are shown for the lymph node population gated on donor cells. CD44 expression by the preinjection donor population (D) is shown for comparison.

Figure 4

Efficient cytolytic effector function does not develop in OT-I cells undergoing homeostatic expansion. OT-I T cells were transferred into irradiated B6 mice and analyzed 5 days later. The mice were (green squares) or were not (red circles) immunized with OVAp. Lymph node cells were tested in a ⁵¹Cr-release assay against EL4 cells pulsed with or without OVAp. The response toward EL4/OVAp targets is shown versus the effector-to-target (E:T) ratio calculated for OT-I cells (see Materials and Methods). Two mice per group were analyzed and the average response is shown with the range indicated by error bars. Lysis by an in vitro-cultured OT-I cytotoxic T lymphocyte (CTL) line was used as a positive control (black triangles). Responses to EL4 with no added peptide were less than 4% for the in vitro CTL line and less than 1% for all other responders.

Figure 5

Expansion of sorted CD44^low “naïve” OT-I T cells is similar to that of unsorted cells. OT-I CD8⁺ donor cells were used as a bulk population or after sorting for low CD44 expression. Reanalysis of these populations (A) revealed the unsorted cells (green line) to be 8.7% CD44^high, whereas the sorted cells (red line) were <0.7% CD44^high, using the marker shown. The unsorted OT-I cells (B and D) and sorted CD44^low (C and E) population were transferred into irradiated B6 (red lines) or TAP^0/0 (blue lines) and assayed for CFSE expression (B and C) and CD44 staining (D and E) at day 10 after transfer. Data are representative of 2–3 mice per group.