The early stages of the recall response are also mediated by memory CD8⁺ T cells that migrate into the lung from the circulation in response to inflammatory signals (phase 2 cells). Despite the fact that these cells are not proliferating and therefore cannot maintain a sustained response, they nevertheless make a substantial contribution to the overall recall response (Ely et al., 2003a). These cells appears to be derived predominantly from the circulating effector memory T cell pool (Hikono et al., 2006). To determine the effect of age on the contribution of phase 2 cells to the recall response we determined the numbers of antigen-specific (tetramer-positive) memory CD8⁺ T cells that could be recruited to the lung airways by a heterologous virus infection. As shown in Table 1, the numbers of memory T cells that could be recruited to the lung airways declined dramatically with time. By 20 months post-infection, there was essentially no recruitment of these cells, indicating that phase 2 was completely missing from the recall response at this time. Interestingly, this progressive decline in the capacity to recruit phase 2 effector-memory CD8⁺ T cells directly correlates with a progressive loss of effector-memory T cells from the splenic memory T cell pool (Roberts et al., 2005). Although the numbers of memory cells in the spleen remains relatively constant, the composition of the population switches from a predominantly effector-memory population (which can be readily recruited to the lung) to a predominantly central-memory population (which cannot be recruited to the lung). The progressive loss of phase 1 and phase 2 memory cells over time corresponds to a decline in the efficacy of cellular immunity to respiratory virus infections.

The mechanisms underlying the increasing proliferative efficacy of memory T cell subpopulations over time are not known. Although it is possible that all of the memory cells (on a per cell basis) gradually improve their capacity to proliferate in response to antigen, it seems more likely that there is simply the progressive accumulation of a specific subpopulation of cells with strong proliferative potential. Since central-memory (CD62L^hi) CD8⁺ T cells progressively accumulate in the spleen over time, we speculated that these cells may have a superior proliferative response compared to effector-memory T cells. For example, if the CD62L^hi cells had a greater proliferative capacity than CD62L^lo cells, their progressive accumulation in the spleen could account for the increase in responsiveness of the total memory pool. However, dual adoptive transfer studies with isolated CD62L^hi and CD62L^lo populations revealed that both populations increased their capacity to contribute to the recall responses over time. Thus, the increase in recall efficacy of CD8⁺ T cells over time cannot be entirely attributed to central-, or effector-memory phenotype and must correlate with some other feature of the memory T cell pool. One possibility is that the proliferative capacity correlates with changes in the TCR repertoire and the overall avidity of the cells for antigen. However, we did not observe differences in the functional avidity of antigen-specific T cells in aged mice (Cole et al., 1994, and data not shown). An alternative possibility is that proliferative capacity correlates with memory CD8⁺ T cell subsets that differ in activation status. Although memory CD8⁺ T cell are resting, they express different patterns of activation markers such as CD43 and CD27 and the distribution of these subpopulations changes over time. Thus, memory CD8⁺ T cells in the spleen progressively change from a more activated (CD43^hi) phenotype to a less activated (CD43^lo) phenotype. Preliminary dual-adoptive transfer studies suggest that proliferative efficacy may indeed be linked to low levels of CD43 expression. If this turns out to be correct, the increasing efficacy of the systemic recall response can be attributed to the accumulation of memory cells with the most resting phenotype. Clearly, more needs to be done to understand the differences in recall efficacy of systemic memory CD8⁺ T cells.