In summary, these studies demonstrate rapid responsiveness of a local population of CD4+
T cells in the lung airway at the time of secondary infection. Airway cells respond 24-48 hours prior to the appearance of divided secondary effectors, and turn on an inflammatory response program that promotes reduced viral titers and may help shape the developing innate response. After the first two days of infection, a second phase of divided cells can be detected that differ in both functional and cell surface phenotypes. This report, in conjunction with previous reports demonstrating antigen non-specific accumulation of primed T cells during infection 
, provides further support for the multi-phase model of adaptive immune responsiveness to secondary infection.
In order to understand the overall immune response to secondary contact with a pathogen, it is important to first identify when and where the different populations of immune cells participate in the response. Here we have carefully documented the reactivation, timing, phenotype, and gene expression profiles of the flu-specific memory CD4+
T cells in the airways. We have previously shown that the CD4+
T cells in the airways of immune mice are functionally distinct from those in the lymphoid organs and are comprised of at least two populations differentiated by alpha-1 integrin expression and reactivation potential 
. Mice that are deficient in alpha-1 integrin demonstrate increased susceptibility to secondary virus challenge 
but have no defects in a primary response to non-lethal influenza challenge. The diminished protection has been associated with a reduction in the number of virus-specific memory T cells present in the lung at the time of re-challenge, and is also observed in wild-type mice after longer time intervals 
. Although much of this loss of protection has been attributed to cytotoxic CD8+
T cells 
, local CD4+
memory T cells are also present. The data presented herein shows that these cells are activated and participate in the response, potentially by modifying the inflammatory response and/or directly contributing to reduction in lung viral titer.
One outstanding question not addressed in this report is the proportion of airway memory T cells that respond to secondary infection in vivo
. While a high proportion of virus-specific T cells respond in ex vivo
restimulation, whether reactivation is uniform in vivo
remains to be determined. The specific localization of memory cells within the lung and airway environment may be important. For example, data from our laboratory suggest airway CD4+
cells expressing the α1β1 integrin VLA-1 may be uniquely positioned for secondary responsiveness due to the capacity of VLA-1+
cells to localize near large airways 
. Presumably, localization within the airway epithelium is a critical feature of rapid and effective immune function. While it is easy to appreciate the potential for cytotoxic CD8+
T cells to engage class I MHC on infected epithelial cells, the mechanisms by which CD4+
T cells are reactivated remain less clear. While our data suggests that professional antigen-presenting cells bearing influenza antigen are more numerous in immune mice, it is possible that both antigen-specific responses driven via class II MHC interactions, as well as antigen non-specific responses via inflammatory cytokines 
could result in multiple antigen specific and non-specific T cell populations responding in concert during early infection.
We discovered profound differences in the cellular composition of the lung innate response to secondary infection compared to primary infection. While it seems clear from our data and others 
that local adaptive memory can shape the early tissue response, it is unclear how these changes in the lung affect the outcome of infection. For instance, in analysis of cytokine secretion from endogenous airway memory CD4+
T cells, we found a significant increase in IL-10 expression during secondary infection. This was in association with a decreased recovery of granulocytes compared to primary infection, consistent with published data on Gr-1+
cells in IL-10−/−
mice after bleomycin-induced lung injury or fungal infection 
. In addition, aged mice have been shown to have increased levels of IL-10, which appears to reduce innate responses in the lung 
. These data suggest that increased IL-10 production from CD4+
T cells may limit the granulocyte response during secondary infection. However, depletion of memory CD4+
T cells in our studies not only alters the contribution of memory CD4+
T cells themselves to secondary immune protection, but also the CD4-induced changes in the local innate inflammatory and cellular environment. It would be useful to have a system in which memory CD4+
T cells established by respiratory infection are the only cross-reactive T cell subset, though such a system does not currently exist. With such a system, it may be possible to further segregate the relative contributions of adaptive memory and the altered innate response in protection.
Our results complement a recent report showing increased innate inflammatory cytokine secretion in the lung during secondary infection 
by profiling the contribution of the memory CD4+
T cells themselves to the early inflammatory environment, and suggest that airway memory cells present at the time of infection, rather than circulating memory cells recruited to the lung, are responsible for the early changes observed. We also show that, coupled with changes in innate inflammatory cytokines, innate cell recovery is significantly altered by the presence of an adaptive tissue response, and may be the result of CD4+
T cell reactivation. We propose that rapidly responding extralymphoid tissue memory cells have the capacity to shape changes in the composition of the local innate response that result in a greater capacity for viral clearance and regulation of immune pathology. Further investigation into the protective and pathogenic roles of extralymphoid memory cells will give greater understanding of their importance in immune protection.