Complement receptor-mediated uptake and tumor necrosis factor-α-mediated growth inhibition of Mycobacterium tuberculosis by human alveolar macrophages

The relative phagocytosis and intracellular fate of Mycobacterium tuberculosis (MTB) (H37Ra) in human alveolar macrophages (AM) and their precursors blood monocytes (MN) was investigated. Uptake of MTB by MN and AM was confirmed by electron microscopy. At an infection ratio of 100:1 (MTB:target cell), the percentage of infected AM and the number of MTB per AM was > MN (p <0.001, p <0.0001, respectively). Uptake of MTB was increased by increasing concentrations of serum and decreased in the presence of heat- inactivated serum. Among complement receptors (CR) CR1, CR3, and CR4, the major CR mediating uptake of MTB by MN were CR1 and CR3, whereas for AM, CR4 was the major CR. When MN and AM were infected with MTB and cultured for up to 7 days, AM limited intracellular growth of MTB more effectively than MN as determined by a CFU assay. MTB stimulated production of TNF-α by mononuclear phagocytes and by AM > MN (p <0.007). Pentoxifylline inhibited TNF-α production by mononuclear phagocytes and concurrently increased MTB growth (AM > MN). A polyclonal neutralizing antibody to TNF-α also increased MTB growth in AM. Thus, AM are more efficient in phagocytosis of MTB than MN, and uptake is mediated through CR4 to a greater extent than CR1 or CR3. The slowed replication of MTB in AM is associated with an increase in TNF-α production, and intracellular growth is promoted by pentoxifylline and neutralizing antibody to TNF-α. These data suggest that AM may play a prominent and efficient role in the primary defense of the lung in tuberculosis through CR-mediated uptake, predominantly CR4, and TNF-α- mediated killing of MTB.