Early events and implication of F-actin and annexin I associated structures in the phagocytic uptake of Brucella suis by the J-774A.1 murine cell line and human monocytes

Brucella spp. are facultative, intracellular pathogenic bacteria that cause brucellosis, a zoonosis affecting mammalian species. Brucella entry into m yelomonocytic cell lines is highly enhanced by opsonization. Few studies ha ve been undertaken to unravel the first interactions between these bacteria and their host cells. This paper deals with early events following contact of Brucella suis with the J-774A.1 phagocytic cell line and differentiated monocytes. Phagocytic uptake of bacteria was documented under a fluorescen ce microscope using GFP-expressing B. suis. Unlike entry in the J-774A.1 ce ll line, non-opsonized Brucella entered differentiated human monocytes as e fficiently as opsonized bacteria. However, following Ih infections, a mean of only three bacteria were phagocytized and the whole monocyte population was only infected after a 4 h infection. Contact of non-opsonized Brucella with phagocytes did not induce marked structural changes at the cell surfac e, as revealed by scanning electron microscopy. Contact of Brucella (opsoni zed or not) elicited transient local recruitment of F-actin, revealed by ph alloidin labelling, and of annexin I-associated structures, revealed by imm unofluorescence staining. Finally, bacteria appeared to be rapidly internal ized in monocytes once they had adhered to the cell surface. A low percenta ge of infected cells and few adhered and/or internalized bacteria following short-term infections could have resulted either from the fact that there were few sites of entry or the weak bacterial initial interactions with the host-cell membrane or the bacterial receptor. (C) 2000 Academic Press.