After Neisseria gonorrhoeae overcomes the first line of host defense constituted by neutrophils, it encounters secondary host defenses. One host defense mechanism in combating microbial infection is the production of antibodies. In the present study, we showed that activation of human B cells by IL-2 stimulates expression of human CEACAM1, an inhibitory receptor, and the interaction of human CEACAM1 with GC inhibits antibody production. We further found that the GC-CEACAM1 interaction in human peripheral B cells results in induction of cell death.
A question arises as to whether GC interact with lymphocytes in vivo. Although GC infection is basically a local process, it is possible that GC may encounter lymphocytes for the following reasons. The existence of a local immune system in the female genital tract has been demonstrated, with a predominance of IgA-producing plasma cells and T cells in the fallopian tubes, uterine cervix, and vagina (16
). T cells are present in these sites in numbers approximately twofold higher than plasma cells. GC-infected segments of fallopian tubes were shown to contain 6- to 10-fold-increased amounts of plasma cells of all classes (33
). These data suggest that a local immune response may provide a defense against GC infection. A delay in an immune response caused by the interaction of GC with CEACAM1 could potentially increase the chance that bacteria successfully colonize the urethral or cervical mucosa and persist for an extended period of time.
CEACAM1 and CEACAM3 surface antigens serve as receptors for some opacity (Opa) proteins of GC, promoting adherence and invasion of this microorganism (12
) in epithelial cells and neutrophils. GC may exploit a well-characterized intracellular signaling pathway, namely, the ITAM signal transduction pathway, to mediate invasion by interaction with CEACAM3 (5
). On the other hand, CEACAM1 was identified as an inhibitory receptor able to mediate negative signals delivered through its ITIM, which recruits SHP-1 and SHP-2 phosphatases (15
). It has been shown that CEACAM1 directly interacts with SHP-1 and SHP-2 phosphatases (1
). In general, the ITIM is required to inhibit ITAM-promoted activating signals when ITAM- and ITIM-bearing receptors are coactivated and coligated. However, Boulton and GrayOwen showed that N. gonorrhoeae
cells inhibit CD4+
T-cell proliferation, which can involve the recruitment of SHP-1 and SHP-2 (8
). Different outcomes may depend on different host cells (CD4+
T cells versus B cells) used in these studies. Our results support the notion that, in order to exert their inhibitory effects, inhibitory receptors should be coligated and costimulated with activating receptors. Otherwise, the so-called inhibitory receptors may deliver an activation signal if they are induced directly. There is a functional difference between CEACAM1-S, which lacks a cytoplasmic region that includes the ITIM, and ITIM-mutated CEACAM1-L-Y459F. As shown in Fig. and described previously, we could not isolate transfectants with high expression of CEACAM1-S, but DT40 cells tolerate high levels of CEACAM1-L-Y459F, suggesting that another functional domain(s), in addition to the ITIM, is missing in CEACAM1-S.
CEACAM1 behaves like a cell death molecule. The question remains as to which signaling pathways are involved. Direct activation of CEACAM1 by GC does not involve SHP-1, SHP-2 (Fig. ), Syk kinase, or phospholipase C activity (data not shown) but partially depends on SHIP and the BTK kinases in DT40 cells. SHIP, by hydrolyzing PIP3
[phosphatidylinositol (3, 4, 5) trisphosphate], leads to the association of BTK (6
). BTK family kinases have been shown to play an important role in the regulation of various cellular processes, including apoptosis and cell motility (43
). Direct stimulation of CEACAM1 by GC may stimulate a cell death pathway. The activities and functions of CEACAM1 mimic another receptor, FcγRIIB, a well-documented inhibitory receptor able to stimulate the death of DT40 B cells (43
) when activated directly. FcγRIIB-mediated cell death also does not involve its ITIM motif but rather its association with SHIP and BTK. Although our functional studies indicated that the same may be possible for CEACAM1, the direct association of CEACAM1 with SHIP or BTK remains to be determined. Nevertheless, these data show that the cellular responses initiated through CEACAM1 resemble the events following activation of FcγRIIB either by coligation for inhibitory activation (6
) or direct activation for apoptotic ability demonstrated in this study. In original reports, in which binding of measles virus with FcγRIIB on B cells inhibits antibody production, the primary reason appears to be the killing of B cells by this virus (45
). Very recently, the short form of CEACAM1 was demonstrated to mediate apoptosis and revert mammary carcinoma cells to a normal morphogenic phenotype (30
), indicating again that CEACAM1 can function as cell death receptor and that it does not require the ITIM for that activity. Finally, it should be stated that CEACAM1-mediated cell death is dependent on which types of cells are activated, since neither forms of CEACAM1 stimulate death of HeLa cells, even when challenged with GC. CEACAM1 did not promote invasion of bacteria in DT40 cells (data not shown).
Repeated infections among gonorrhea patients are very common. GC appear to have the capability to inhibit the immune response, which may be in part due to suppression of antibody production (22
). The present study begins to unveil potential mechanisms of antibody suppression, i.e., GC specifically targeting the CEACAM1-expressing B cells and causing cell death, which consequently may play a role in GC-induced immunosuppression during infection. It is of interest that GC use the same molecule, CEACAM1, not only for their binding to host but also for subversion of regulatory immune pathways. Further studies should uncover mechanisms of gonococcal infection and help us understand how microbial pathogens exploit host cells.