Mycoplasma respiratory disease is immunopathologic 
. It is clear that elements of the adaptive immune response contribute to the pathology, while some responses are protective against M. pulmonis
infections. Pulmonary T cell activation, and the mechanisms that regulate these responses, is clearly instrumental in the pathogenesis of mycoplasma respiratory disease of the lower respiratory tract 
. Because of their central role in the activating and regulating T cell responses, APC likely participate in determining whether immune-mediated pathology or protection develops due to mycoplasma respiratory infection, but there is little information on the role of APC populations, particularly DC, during generation of immune and inflammatory responses in mycoplasma respiratory disease.
The numbers of DC and macrophages in the lungs of mice increased during the pathogenesis of mycoplasma respiratory disease. There was about a 3- to 4-fold increase in the numbers of CD11c+
DC and F4/80+
macrophages in the lungs of mice by 14 days after infection. There was also a significant increase in the numbers of double positive (CD11c+
) cells, which were found to represent alveolar macrophages in another report 
. The increase in CD11c+
cells is consistent with studies demonstrating increasing numbers of DC in lungs in other inflammatory diseases 
. The major population of DC that increased expressed CD11b, consistent conventional DC. Additionally, the increases in APC populations due to mycoplasma infection were not apparent at 7 days after infection (data not shown). Our previous studies 
similarly demonstrate that the numbers of CD4+
Th cells and CD8+
T cells increase in lung between day 7 and 14 after infection. Thus, there is a concomitant increase in pulmonary DC and macrophage numbers with T cell responses after mycoplasma infection. Furthermore, this time frame coincides with the development of the chronic inflammatory response of the airways in response to the infection. Taken together, there are indeed changes in CD11c+
DC and macrophage populations in lungs after mycoplasma infection that accompany development of T cell-mediated inflammation, suggesting a role for these APC in maintaining and modulating T cell responses during the pathogenesis of pulmonary inflammatory disease due to mycoplasma infection.
As the numbers of DC and macrophages increased, there was a change in the populations of DC and macrophages during disease pathogenesis. By 14 days post-infection, expression of MHC II was higher on CD11c+
DC, as well as a greater percentage of DC were CD40+
. Co-stimulatory molecules CD80 and CD86 were also expressed in conjunction of MHC II, especially on CD11+
DC cells. There were differences in the expression MHC II, CD40 and CD86 between the naïve APC populations, and changes in expression of these markers suggested that at least a portion of each of the cell populations responded to the infection. In support of activation of APC in lungs in response to infection, there were differences in cytokine mRNA profiles in both CD11c+
DC and F4/80+
macrophages isolated from lungs of naïve (control) and infected mice. After infection, both DC and macrophage populations also showed an increased expression of cytokine or chemokine mRNA's involved in inflammation (e.g. Nos2, Spp1, TNF, Ltb, MIF, IL-6 and IL-1α), and interestingly they had increased expression of cytokine mRNA's that can function in the recruitment and activation of T cells (e.g. CCL4, CCL8, CxCL11 and IL-15). In fact, previous studies from our lab demonstrated that CCL4 and CCL8 are likely involved in the recruitment of T cells into lesions 
, and the results in the current study suggest that DC and macrophage populations are sources of these chemokines. In any case, it is clear that both macrophages and DC in the lungs are activated or mature as a result of mycoplasma infection, and their differing profiles of cytokine mRNA expression suggest that pulmonary macrophage and DC populations play different roles in modulating immune and inflammatory responses to mycoplasma. In fact, pulmonary CD11c+
DC from mycoplasma-infected lungs were more potent antigen presenting cells than those from control lungs in stimulating mycoplasma-specific T (CD4+
Th) cell responses in vitro
. Although other pulmonary cell populations had some activity in supporting T cell responses, DC from diseased lungs were significantly more effective.
macrophages and CD11c+
DC were localized in the inflammatory airway infiltrates typically associated with mycoplasma respiratory disease. Both cell populations surrounded a central Th cell region of the inflammatory infiltrates. The Th cells in the center were frequently co-localized with CD11c+
cells. Previous studies demonstrate that a population of Th cells mediates the proinflammatory responses in lungs 
. The current results demonstrate that CD11c+
DC are potent stimulators of mycoplasma-specific Th cell responses in vitro
, and DC co-localize with Th cells in the pulmonary lesions of mice infected with mycoplasma. In ongoing studies, the priming of mice with mycoplasma Ag-pulsed DC stimulates Th cell responses in vivo
and results in more intense lymphoid infiltration into the lungs after subsequent infection with mycoplasma (N. Dobbs, X. Zhou, M. Pulse, L.M. Hodge and J.W. Simecka, Unpublished results). Although CD11c+
DC were shown to also support mycoplasma-specific CD8+
T cell activation, there were relatively few CD8+
T cells located within the inflammatory infiltrates and interactions between these cells with either CD11c+
DC or macrophages were not consistently found. Thus, DC-Th cell interactions in the lung appear to occur in vivo
, and these studies suggest that DC facilitates the activation of effector Th cells that contribute to the inflammatory responses associated with mycoplasma respiratory disease.
Overall, the present studies demonstrate that during the pathogenesis of mycoplasma lung disease the populations of DC and macrophages shift from an “inactive” to an “active or mature” state. After infection, DC from the lungs of infected mice became most capable of stimulating mycoplasma-specific T cell responses in vitro, and DC co-localized with Th cells in inflammatory infiltrates in the lungs of infected mice. Thus, DC does appear to be major APC population responsible for T cell stimulation in the lungs of mycoplasma-infected mice, and these in vivo interactions likely contribute to the immune responses that impact disease pathogenesis. Among these DC, conventional DC are the major contributer to the immune responses. Although macrophages are unlikely to be as effective in directly stimulate T cell responses, macrophages are also associated with the inflammatory lesions and are localized primarily on the periphery of the cellular infiltrates. These results suggest that macrophages do have a significant but different impact from DC on the generation and/or maintenance of host responses associated with mycoplasma disease through the production of cytokines and other factors. Further studies are needed to elucidate the role of each of these cell populations and the cytokines produced in the pathogenesis of mycoplasma disease. Information gained from these studies provides important insight into the immune mechanisms involved in disease pathogenesis and will likely support the development of effective vaccines against mycoplasma respiratory diseases.