Protective niche for Borrelia burgdorferi to evade humoral immunity

Abstract

The Lyme disease spirochete, Borrelia burgdorferi, is an extracellular microbe that causes persistent infection despite the development of strong immune responses against the bacterium. B. burgdorferi expresses several ligand-binding lipoproteins, including the decorin-binding proteins (Dbps) A and B, which may mediate attachment to decorin, a major component of the host extracellular matrix during murine infection. We show that B. burgdorferi was better protected in the joints and skin, two tissues with a higher decorin expression, than in the urinary bladder and heart, two tissues with a lower decorin expression, during chronic infection of wild-type mice. Targeted disruption of decorin alone completely abolished the protective niche in chronically infected decorin-deficient mice but did not affect the spirochete burden during early infection. The nature of protection appeared to be specific because the spirochetes with higher outer surface protein C expression were not protected while the protective niche seemed to favor the spirochetes with a higher dbpA expression during chronic infection. These data suggest that spirochetal DbpA may interact with host decorin during infection and such interactions could be a mechanism that B. burgdorferi uses to evade humoral immunity and establish chronic infection.

Figure 1

B. burgdorferi is better protected against immune clearance in the joints and skin than the bladder and heart. Twenty SCID and 10 wild-type mice were infected with cultured B. burgdorferi. Ten of the SCID mice (A) and all of the wild-type mice (B) were sacrificed 2 months later. The rest of the SCID mice (C) were passively immunized with anti-Borrelia sera. DNA samples were prepared from bladder, heart, joint, and skin tissues and quantified for flaB and actin DNA copy numbers by qPCR. The data are expressed as flaB DNA copy numbers per 10⁶ actin DNA copies.

Figure 2

Tissue differential decorin expression. RNA samples were prepared from the bladder, heart, joints, and skin of the wild-type mice that had been used to generate data for Figure 1B, converted to cDNA, and analyzed by qPCR for actin and decorin mRNA copy numbers. The data are expressed as decorin mRNA copy numbers per 10³ actin mRNA copies.

Figure 3

Targeted disruption of decorin abolishes the protective niche for B. burgdorferi. Fifteen Dcn^+/+ (WT) and 15 Dcn^−/− mice (deficient) were infected with cultured B. burgdorferi and sacrificed 2 months later. DNA samples were prepared from bladder, heart, joint, and skin tissues and quantified for flaB and actin DNA copy numbers by qPCR. The data are expressed as flaB DNA copy numbers per 10⁶ actin DNA copies.

Figure 4

Spirochetes with higher dbpA expression are better protected in the joints of wild-type mice. RNA samples were prepared from the bladder, heart, joints, and skin of the mice that had been used to generate data for Figure 3, converted to cDNA, and analyzed by qPCR for flaB and dbpA mRNA copy numbers. The data are expressed as dbpA mRNA copy numbers per 10³ flaB mRNA copies.

Figure 5

Decorin deficiency does not affect early infection. Thirty Dcn^+/+ (WT) and 30 Dcn^−/− mice (deficient) were infected with host-adapted B. burgdorferi. Half of them were sacrificed 2 weeks later (A) and the rest were euthanized 2 months after infection (B). DNA samples were prepared from the bladder, heart, joint, and skin tissues and examined for the tissue spirochetal burden by qPCR. The data are expressed as flaB DNA copy numbers per 10⁶ actin DNA copies.

Figure 6

The protective niche does not protect ospC-expressing spirochetes. RNA samples were prepared from the bladder, heart, joints, and skin of the SCID mice (A) that had been used to generate data for Figure 1A, and the wild-type and Dcn^−/− mice (B) that had been used to generate data for Figure 5, converted to cDNA, and analyzed by qPCR for flaB and ospC mRNA copy numbers. The data are expressed as dbpA mRNA copy numbers per 10³ flaB mRNA copies.