Intracellular trafficking and killing of Streptococcus pneumoniae by humanalveolar macrophages are influenced by opsonins

Human alveolar macrophages (HAM) are the major resident phagocytic cells of the gas-exchanging areas of the lung. Following contact with macrophages, bacteria enter phagosomes, which gradually acquire the characteristics of t erminal phagolysosomes, with incorporation of lysosome-associated membrane protein (LAMP). We measured the binding of type I Streptococcus pneumoniae to the surface of HAM and then measured subsequent internalization and phag osomal incorporation of LAMP-1 under various opsonic conditions. Opsonizati on with serum containing immunoglobulin resulted in significantly greater b inding of pneumococci to HAM compared with opsonization with immunoglobulin G (IgG)-depleted serum containing complement, which in turn resulted in ma rginally increased binding over that observed in the absence of opsonizatio n. Internalization of opsonized S. pneumoniae gradually increased to a maxi mum of 20% of bound bacteria by 120 min of warm incubation, with 20% of int ernalized pneumococci being localized within LAMP-containing compartments b y 80 min. Internalization of opsonized S. pneumoniae by HAM correlated with a reduction of bacterial viability. When inocula were adjusted so that pne umococcal binding under different conditions was equalized, subsequent inte rnalization, trafficking to LAMP-containing compartments, and reduction of bacterial viability were less efficient in the absence of opsonization than that observed following opsonization with adsorbed or IgG-replete adsorbed serum, Once bound to the surface of HAM, pneumococci opsonized with adsorb ed serum with or without Ige were internalized, processed, and killed equal ly well. In conclusion, binding, intracellular trafficking, and killing of S. pneumoniae by HAM are each significantly increased by opsonization with serum containing immunogloblin and/or complement.