This thesis aimed to investigate the role of alveolar epithelial cells (AEC) on immune responses against mycobacterial infections, specifically, the role of AEC in modulating macrophage functions through the secretion of broad variety of factors.

In paper I, we compared murine AEC with interstitial macrophages (PuM) in their ability to take up and control mycobacterial growth and their capacity as antigen-presenting cells. We found that AEC were able to internalize and control bacterial growth and present antigens to T cells from immunized mice. In addition, both AEC and PuM exhibited distinct patterns of secreted factors, and a more comprehensive profile of AEC responses revealed that AEC were able to secrete different factors important to generate various effects in other cells. Paper II: Since AEC secrete a broad variety of factors, we hypothesized that being in the interface; AEC may play an important role in transmitting signals from the external to the internal compartment and in modulating the activity of PuM. Thus, we prepared AEC-derived media and tested their effect on bacteria and a number of macrophage functions a) migration, b) phagocytosis and control of intracellular bacterial growth, and c) alteration in cell morphology and expression of surface markers. We found that AEC-secreted factors had a dual effect, in one hand controlling bacterial growth and on the other hand increasing macrophage activity. In paper III, we first investigated the responsible mechanisms of intracellular bacterial growth control mediated by AEC-derived media. We found that infected macrophages upon AEC-secreted factors increased the control of intracellular bacterial growth by iNOS-independent pathways. Compared with other macrophage types, PuM, did not control the intracellular bacterial growth upon the well-known potent macrophage activator, IFN-γ. We found that SOCS1 was involved in the un-responsiveness to IFN-γ by PuM to control the intracellular bacterial growth. We suggested that PuM are restricted in their inflammatory responses perhaps for avoiding tissue damage.

Overall, the current findings highlight the importance of AEC in the defense against bacterial infection in the lungs by secreting factors involved in activation and differentiation of immune cells such as macrophages.