Primary syphilis — transmission, adhesion, local host immune response.
is usually transmitted sexually through microabrasions in mucosal membranes or skin and rapidly enters the bloodstream to disseminate to other tissues. T. pallidum
can be identified by PCR in the bloodstream of patients with all stages of syphilis, and the quantity of treponemes in blood is highest during early syphilis (2
). Individuals with lesions of early syphilis are most likely to transmit T. pallidum
. While the risk of infection in exposed individuals is approximately 30% (range, 10%–80%) (4
), inoculation studies with the Nichols strain of T. pallidum
suggest that the intradermal ID50
is only 57 organisms (7
). The natural history of T. pallidum
infection is summarized in Figure .
The natural history of untreated syphilis in immunocompetent individuals.
To establish infection, T. pallidum
adheres to epithelial cells and extracellular matrix components of the skin and mucosa. Several T. pallidum
proteins mediate adherence, including TP0155 and TP0483, which bind to matrix fibronectin and to both soluble and matrix forms of fibronectin, respectively (8
). TP0136, a protein identified by reactivity with primary human syphilitic sera (9
), also binds to human fibronectin (10
). TP0751 can bind to laminin, which has the highest concentration in the basement membrane (11
), and to fibrinogen, a blood-clotting protein that functions to contain bacteria (13
). TP0751 can also degrade laminin and fibrinogen using its zinc-dependent protease domain, which may be a means by which T. pallidum
disseminates to surrounding tissues and the bloodstream (13
replicates at the site of initial inoculation, dividing once every 30–33 hours (14
), inducing a local inflammatory response that results in a painless chancre approximately 3–6 weeks after initial infection. In each chancre, proliferating spirochetes are surrounded by immune cells, including CD4+
T cells, plasma cells, and macrophages, which produce IL-2 and IFN-γ cytokines, indicating a Th1-skewed response (16
). Tissue necrosis and ulceration occur due to small vessel vasculitis, and trafficking immune cells cause a non-tender regional lymphadenopathy. Within 3–8 weeks, the chancre heals, indicating clearance of T. pallidum
locally. However, by this time, T. pallidum
has spread systemically to multiple tissues and organs, setting the stage for secondary syphilis.
Secondary syphilis — motility, systemic host immune response, diagnosis, systemic spread.
propels itself using a corkscrew-like mechanism by rotating around its longitudinal axis, using endoflagella contained within the periplasmic space between the cytoplasmic membrane and the outer membrane (22
). T. pallidum
traverses the tight junctions between endothelial cells (25
) to enter the perivascular spaces, where large numbers of treponemes and immune cells accumulate. Based on electron microscopy images of secondary syphilis skin lesions, T. pallidum
may also use transcytosis to spread through the endothelium (27
). T. pallidum
can induce the production of MMP-1 (28
), which degrades collagen and may facilitate access to and egress from the bloodstream, resulting in systemic spread.
Usually within 3 months of infection, symptoms of secondary syphilis appear. The most common clinical manifestation is a disseminated maculopapular rash. Additional symptoms may include malaise, weight loss, muscle aches, generalized lymphadenopathy, patchy alopecia, meningitis, ocular inflammation, mucous patches (localized inflammation of mucosal tissues in the oral cavity and genitals), hepatitis, and gastric dysmotility (29
), reflecting T. pallidum
invasion and the resulting immune cell infiltration of these tissues.
Although T. pallidum
has structural similarities to classical Gram-negative bacteria, such as having outer and inner membranes and a periplasmic space, it lacks lipopolysaccharide, a potent proinflammatory glycolipid, and does not produce any known toxic proteins. Therefore, most of the symptoms and tissue damage related to syphilis are due to activation of the host inflammatory and immune responses. Exposure to whole T. pallidum
and its lipoprotein TpN47 can induce expression of the adhesion molecules ICAM-1, VCAM-1, and E-selectin (25
), which are important in adhesion of immune cells to vascular endothelium for migration into sites of infected tissue. Patients with secondary syphilis have a local immune response in the skin, consisting of monocytes, macrophages, CD4+
T cells, and DCs (32
). This proinflammatory response is due to the lipid moiety contained on the many lipoproteins of T. pallidum
). Early syphilis lesions transiently contain scant polymorphonuclear leukocytes (PMNs) (37
), and injection of recombinant T. pallidum
lipoproteins TpN17 (TP0435) and TpN47 (TP0574) into the dermis can induce transient infiltration of PMNs (35
), as well as a local enrichment of monocytes, macrophages, memory T cells, and DCs (38
The interaction of TpN47 with TLR2 on the surface of macrophages induces the production of IL-12 (40
). When DCs are exposed to T. pallidum
or purified TpN47, they release inflammatory cytokines, such as IL-1β, IL-6, IL-12, and TNF-α (41
), and express maturation markers, including CD54, CD83, and MHC class II (38
). T. pallidum
lipoproteins also stimulate macrophages and DCs by binding CD14, which transmits activation signals through the TLR1/TLR2 heterodimer (40
). The miniferritin TpF1 stimulates human monocytes to release IL-10 and TGF-β, which are key cytokines that promote Treg differentiation and may also allow long-term persistence of T. pallidum
in the human host (46
T cells present in the skin colocalize with staining for IFN-γ, perforin, and granzyme B (32
), as well as IL-17 (34
). Studies of lesional skin samples from patients with secondary syphilis show that plasma cells appear later (34
The humoral immune response produces antibodies that function in opsonization (47
) and complement-mediated immobilization or neutralization (48
). Macrophages clear T. pallidum
from sites of infection through phagocytosis of opsonized organism (47
) using both IgG and IgM antibodies (51
). A study using an array of 882 polypeptides predicted to be in the T. pallidum
proteome identified 106 proteins that could induce a detectable antibody response (53
). Two T. pallidum
lipoproteins that induce high titers of antibodies are TpN17 and TpN47 (54
), both of which are used in new enzyme and chemiluminescence immunoassay (EIA/CIA) serological tests for syphilis. Genome analysis of T. pallidum
predicts that there are as many as 22 putative lipoproteins in the organism (1
Measurement of antibodies is important for screening and diagnosis of syphilis. Two categories of antibodies — termed “non-treponemal,” which are directed against phospholipids, and “treponemal,” which are directed against T. pallidum polypeptides — have been used for this purpose. The non-treponemal antibodies are detected by the rapid plasma reagin (RPR) test, the Venereal Disease Research Laboratory (VDRL) test, and the toluidine red unheated serum test (TRUST). Treponemal antibodies are detected by immunofluorescence in the fluorescent treponemal antibody-absorbed (FTA-ABS) test or by agglutination in the T. pallidum hemagglutination (TPHA) or T. pallidum particle agglutination (TP-PA) test.
Traditionally, T. pallidum
infection has been diagnosed using a non-treponemal screening test, with reactive results confirmed using treponemal serologic tests. Rapid point-of-care tests (58
), EIAs (59
), and CIAs (60
) have been developed that detect anti-treponemal IgM and IgG antibodies, usually to recombinant T. pallidum
proteins. The EIA/CIA tests can be automated, which has led some large laboratories in the United States to use revised syphilis screening algorithms beginning with a treponemal test. Positive tests are subsequently confirmed with a non-treponemal test, and discordant sera must be retested with a traditional treponemal test. One disadvantage of these newer tests is that they cannot distinguish between recent and remote syphilis, or between treated and untreated infection. In addition, because the new EIA/CIA tests are more sensitive than the fluorescence or agglutination tests, many sera that are reactive in the EIA/CIA tests are nonreactive in the confirmatory non-treponemal tests, particularly in low-risk populations such as pregnant women (62
). These results have led to concerns about the specificity of the antigens used in these tests for syphilis infection. Indeed, a published study (63
) reports that persons with periodontal disease carry oral treponemes that can be detected with monoclonal antibodies to the same TpN47 antigen used in many EIA/CIA tests. Persons with periodontal disease have detectable antibodies to this and other T. pallidum
antigens. Additional related concerns regarding screening with automated treponemal tests include increased health care and public health costs caused by follow-up of unconfirmed EIA/CIA screening.
In the rabbit model, despite the presence of functional antibodies, passive immunization with immune serum fails to provide protective immunity against T. pallidum
), demonstrating that cellular immunity is also required for protection. The link between humoral and cellular immunity in humans is indicated in studies of human PBMCs exposed in vitro to T. pallidum
: internalization of treponemes by macrophages is facilitated by human syphilitic serum, leading to secretion of TNF-α, IL-6, and IL-1β by macrophages and resulting in IFN-γ production by NK cells, NK T cells, and T cells (65
). After most T. pallidum
have been cleared in the rabbit infection model, a few organisms remain and are able to resist macrophage ingestion even in the presence of immune serum (66
), suggesting this subpopulation may be able to avoid opsonic antibody, persisting to cause latent or later stages of infection.
Early CNS invasion and neurological involvement.
While CNS involvement of syphilis infection is classically considered as the tertiary stage of infection, invasion of the nervous system by T. pallidum and neurosyphilis occur within days or weeks of infection. Neurosyphilis is diagnosed by clinical manifestations (see below) as well as by cerebrospinal fluid (CSF) abnormalities such as elevated white blood cell (wbc) count, elevated CSF protein, or reactive CSF-VDRL test. Many affected patients may be asymptomatic in spite of the presence of abnormal CSF.
While most patients with CNS infection appear to control or clear CNS infection by T. pallidum
, the factors underlying the subsequent development of symptomatic neurosyphilis in some patients are not known. Symptomatic and asymptomatic neurosyphilis are more common when the serum RPR titer is 1:32 or greater regardless of HIV status, or in HIV-infected individuals when the peripheral blood CD4+
T cell count is 350 or fewer cells/μl (67
Symptoms of early neurosyphilis may occur during or following the primary or secondary stages of syphilis, especially in HIV-infected individuals (71
) and include meningitis (headache, fever, and stiff neck), visual changes (blurred vision, loss of vision, photophobia, and other signs of ocular inflammation), hearing changes or loss, and facial weakness. Some studies indicate that HIV-infected individuals may have more significant symptoms of neurosyphilis (72
), and HIV-infected individuals who have symptomatic neurosyphilis have more severe CSF abnormalities (70
). Treatment of HIV-infected patients with antiretroviral therapy (ART) decreases the chance of developing neurosyphilis by 65% (70
), suggesting that immune reconstitution with ART may result in an improved local immune response against T. pallidum
and better control of the infection.
Diagnosis of asymptomatic neurosyphilis is complicated by the fact that none of the CSF measures currently used is very sensitive (CSF-VDRL) or specific (CSF wbc, CSF protein). In addition, concurrent HIV infection itself may cause an elevated CSF wbc count or protein concentration. A recently described adjunct diagnostic marker for neurosyphilis is the B cell chemokine CXCL13 (74
Latent and tertiary syphilis — antigenic variation and persistence.
Despite a host immune response that results in effective local clearance of T. pallidum from primary and secondary lesions, treponemes persist in many tissues without causing clinical signs or symptoms. This is termed the latent stage. While T. pallidum may seed the bloodstream intermittently during the latent stage and thus infect a developing fetus during pregnancy, sexual transmission is rare.
How can T. pallidum
“escape” immune detection to cause persistent and later stages of infection? Recent evidence suggests that T. pallidum
organisms may be able to evade the acquired immune response by antigenic variation of bacterial surface proteins, consistent with the resistance to phagocytosis of those select treponemes that survive bacterial clearance of the primary lesion (51
). Antigenic variation is well described in related spirochetes that cause relapsing fever (Borrelia hermsii
) and Lyme disease (Borrelia burgdorferi
), each of which also has a multistage clinical course (75
Although T. pallidum
has few integral outer membrane proteins (23
), bioinformatic approaches have identified several candidates, including members of the family of 12 T. pallidum
repeat (Tpr) proteins (79
). Among Tpr family members, TprK is the best studied. A strong antibody and T cell immune response is elicited against TprK (81
), and immunization with recombinant TprK provides partial immunity against infectious challenge (80
). Antibodies raised against recombinant TprK can opsonize T. pallidum
for phagocytosis by macrophages in vitro (80
). TprK sequences differ substantially between and within individual strains (84
), and this diversity is localized to seven discrete variable regions of the protein, which are predicted to be surface exposed. TprK sequence diversity accumulates following development of acquired immunity in the rabbit model (86
). Molecular studies of TprK
show that new variants arise by segmental gene conversion, with the new sequences coming from a large repertoire of “donor sites” located elsewhere on the chromosome (ref. 88
and Figure ). The resulting changes in exposed variable regions of TprK enable the organism to evade antibody binding and opsonophagocytosis (89
). These TprK variant treponemes survive clearance and persist during chronic latent infection.
Gene conversion as the mechanism of antigenic variation of TprK in T. pallidum.
In some individuals, chronic latent infection can reactivate to cause tertiary syphilis, which occurs years to decades after initial infection and can affect multiple organs. In a retrospective study of patients from Oslo in the pre-antibiotic era, approximately one-third of patients with untreated latent syphilis developed tertiary syphilis (90
). Manifestations may include gumma, cardiovascular syphilis, and tertiary neurosyphilis. In the modern antibiotic era, tertiary syphilis is rarely seen, perhaps due to inadvertent syphilis treatment with antibiotics prescribed for other infections.