Borrelia burgdorferi outer surface protein (osp) B expression independent of ospA

Abstract

The outer surface proteins (Osp) A and B are two important lipoproteins of Borrelia burgdorferi, the Lyme disease spirochete. Extensive in vitro studies indicate that ospB shares a common promoter with ospA and thus these two lipoprotein genes are coordinately transcribed. We show here that B. burgdorferi expresses ospB at much higher levels than ospA during experimental murine infection. The ratio of ospA and ospB mRNA transcripts was 3.5:1 in tick-adapted spirochetes while B. burgdorferi matched every ospA mRNA with up to 70 ospB transcripts during murine infection. This was consistent with the analysis of antibody responses to the two lipoproteins, which showed a more frequent OspB response than OspA during chronic murine infection.

Introduction

The outer surface protein (Osp) A has been one of the most intensively investigated lipoproteins of Borrelia burgdorferi, the causative agent of Lyme disease [1]. It was the first B. burgdorferi antigen found to be protective against infection in experimental models of Lyme disease [2] and was approved by the US Food and Drug Administration for use as a human Lyme disease vaccine [3], [4]. Its crystal structure was solved before that of any other Borrelia protein [5]. This lipoprotein is abundantly expressed by B. burgdorferi during in vitro cultivation and by spirochetes within Ixodes scapularis, the tick vector [6], [7], [8]. OspA may play a critical role in the ability of B. burgdorferi to effectively colonize the tick gut [9]. A fresh blood meal by I. scapularis down-regulates the ospA gene [6], [7], [8] and B. burgdorferi does not generally express this gene during infection of the reservoir host [10]. Most studies show that ospA is usually not expressed during experimental murine infection when spirochetes are transmitted to the mammal by the tick vector [11], [12]. In contrast, when spirochetes are inoculated into mice using a syringe, different results can be obtained, depending on the challenge dose. When larger doses of spirochetes are administrated to mice, an OspA immune response and ospA mRNA can be detected shortly after infection [11], [13]. In contrast, ospA mRNA or antibodies to OspA are not evident when lower doses of spirochetes or host-adapted B. burgdorferi are given to mice [11], [14]. This difference is probably due to the ability of spirochetes to rapidly adapt to the host.

OspA expression in humans is more variable. In general OspA antibodies are not detected during infection and ospA mRNA is difficult to identify [15], suggesting that OspA is not expressed. However, OspA antibodies are sometimes found during early infection, and OspA immune responses may develop in patients with late stage Lyme arthritis [16]. Moreover, OspA antigen [17] and antibody [18] have been identified in the cerebrospinal fluid of patients with neuroborreliosis. These studies suggest that ospA may be expressed, at least transiently, in selected human tissues during infection.

In contrast to OspA, very little is known about OspB. Previous in vitro studies clearly demonstrate that ospB lacks an independent promoter and is transcribed with ospA as a single mRNA unit [19], [20]. Since ospB is located at the downstream of ospA, it cannot be independently transcribed without ospA expression. In contrast, ospA can potentially be expressed without ospB since transcription could terminate before a full length ospA–ospB mRNA is generated. This may be the case when B. burgdorferi is cultivated in vitro or resides in the tick vector where ospA is more abundantly expressed than ospB. However, in a preliminary screening study, we noticed that B. burgdorferi expressed ospB but not ospA in murine infected skin [21]. In the current study by using quantitative reverse transcription polymerase chain reaction (qRT-PCR), we carefully investigated the expression of these two genes in different tissues of immunocompetent and severe combined immunodeficient (SCID) mice, confirming the differential expression of these two lipoproteins by B. burgdorferi during murine infection.