Differential Requirements by CD4+ and CD8+ T Cells for Soluble and Membrane TNF in Control of Francisella tularensis Live Vaccine Strain Intramacrophage Growth

During primary infection with intracellular bacteria, the membrane-associated form of TNF provides some TNF functions, but the relative contributions during memory responses are not well-characterized. In this study, we determined the role of T cell-derived secreted and membrane-bound TNF (memTNF) during adaptive immunity to Francisella tularensis live vaccine strain (LVS). Although transgenic mice expressing only the memTNF were more susceptible to primary LVS infection than wild-type (WT) mice, LVS-immune WT and memTNF mice both survived maximal lethal secondary Francisella challenge. Generation of CD44high memory T cells and clearance of bacteria were similar, although more IFN-γ and IL-12(p40) were produced by memTNF mice. To examine T cell function, we used an in vitro tissue coculture system that measures control of LVS intramacrophage growth by LVS-immune WT and memTNF-T cells. LVS-immune CD4+ and CD8+ T cells isolated from WT and memTNF mice exhibited comparable control of LVS growth in either normal or TNF-α knockout macrophages. Although the magnitude of CD4+ T cell-induced macrophage NO production clearly depended on TNF, control of LVS growth by both CD4+ and CD8+ T cells did not correlate with levels of nitrite. Importantly, intramacrophage LVS growth control by CD8+ T cells, but not CD4+ T cells, was almost entirely dependent on T cell-expressed TNF, and required stimulation through macrophage TNFRs. Collectively, these data demonstrate that T cell-expressed memTNF is necessary and sufficient for memory T cell responses to this intracellular pathogen, and is particularly important for intramacrophage control of bacterial growth by CD8+ T cells.