Treponema pallidum Lipoprotein TP0435 Expressed in Borrelia burgdorferi Produces Multiple Surface/Periplasmic Isoforms and mediates Adherence

Abstract

The ability of Treponema pallidum, the syphilis spirochete to colonize various tissues requires the presence of surface-exposed adhesins that have been difficult to identify due to the inability to culture and genetically manipulate T. pallidum. Using a Borrelia burgdorferi-based heterologous system and gain-in-function approach, we show for the first time that a highly immunogenic lipoprotein TP0435 can be differentially processed into multiple isoforms with one variant stochastically displayed on the spirochete surface. TP0435 was previously believed to be exclusively located in T. pallidum periplasm. Furthermore, non-adherent B. burgdorferi strain expressing TP0435 acquires the ability to bind to a variety of host cells including placental cells and exhibits slow opsonophagocytosis in vitro similar to poor ex vivo phagocytosis of T. pallidum by host macrophages reported previously. This phenomenon of production of both surface and periplasmic immunogenic lipoprotein isoforms has possible implications in immune evasion of the obligate pathogen T. pallidum during infection.

Treponema pallidum, the syphilis spirochete: making a living as a stealth pathogen

“…For those antibodies that are potentially opsonic, the extremely low copy numbers of the target proteins likely limits their accessibility, as evidenced by the sizable percentage of treponemes that fail to become surface-labeled or opsonized by high-titer recombinant antisera against TprC/D or BamA 59,65 . Similarly, recent studies suggest that the TP0750–TP0751 protease complex 74,75 and TP0435 78 are expressed at low levels on the surfaces of some treponemes and, consequently, of limited availablity to antibody binding. Two distinct mechanisms for modulating antibody reactivity with Tprs have been proposed.…”

“…The lipoprotein TP0136 is a fibronectin-binding adhesin with sequence heterogeneity among T. pallidum strains (Table 1) 76,77 . Most recently, elegant gain of function experiements using B. burgdorferi as a surrogate genetic host have shown that the lipoprotein TP0435 (Tpp17) functions as a cytadhesin 78 .…”

Syphilis

“…The X-ray structure of TP0751, demonstrating an unusual lipocalin fold, should inform efforts to clarify its multi-functionality 79 . Additionally, the lipoprotein Tpp17 (also known as TP0435) has been shown to be at least partially surface-exposed and can function as a cytadhesin 82 . The structurally characterized lipoprotein TP0453 attaches to the inner leaflet of the outer membrane via its N-terminal lipids and two amphipathic helices within its protein moiety 83 .…”

“…The picture emerging from our evolving concepts of the spirochete’s molecular architecture is multi-factorial and likely involves copious production of antibodies against subsurface lipoprotein ‘decoys’ 57,110 ; poor target availability owing to low copy numbers of outer membrane proteins and surface-exposed lipoproteins 67,77,82,84,93 ; in the case of bipartite outer membrane proteins, limited production of antibodies against surface-exposed epitopes along with skewed production of antibodies against periplasmic domain 84,93 ; organism-to-organism variation in the levels of expression of outer membrane proteins and outer surface lipoproteins through a variety of mechanisms, including phase variation 82,92,115,116 ; and, in the case of TprK, antigenic variation as a result of intra-genomic recombination 89,92,117 . Additionally, the ability of motile spirochetes to ‘outrun’ infiltrating phagocytes and reach sequestered locations, including the epidermis, could be an under-appreciated aspect of immune evasion 10,102 .…”

“…Shown in the outer membrane are TP0751 (as known as pallilysin) 79,81 and Tpp17 (also known as TP0435) 82,241 — two surface-exposed lipoproteins; TP0453, a lipoprotein attached to the inner leaflet of the outer membrane 83 ; BamA (also known as TP0326) 84,94 ; a full-length T. pallidum repeat (Tpr) attached by its N-terminal portion to the peptidoglycan 93,94 ; and a generic β-barrel that represents other non-Tpr outer-membrane proteins identified by computational mining of the T. pallidum genome 112 . Substrates and nutrients present in high concentration in the extracellular milieu (such as, glucose) traverse the outer membrane through porins, such as TprC.…”