Mycoplasma pneumoniae as an Under- Recognized Agent of Vasculitic Disorders

Mycoplasma pneumoniae, commonly known as a major causative agent of primary atypical pneumonia, also causes various kinds of extrapulmonary manifestations involving almost all the organs of the human body. The author has classified the extrapulmonary manifestations due to M. pneumoniae infection into three categories: the first is a direct type in which locally induced cytokines play a role, the second is an indirect type in which immune modulation such as autoimmunity plays a role, and the third is a vascular occlusion type in which vasculitis and/or thrombosis with or without systemic hypercoagulable state plays a role [Narita, 2009, 2010]. This classification system is intended to facilitate the understanding of the pathogenesis of extrapulmonary manifestations due to M. pneumoniae infection. A diagram depicting the possible ways in which M. pneumoniae can induce these three types of extrapulmonary manifestations in relation to the possible pathomechanism of pneumonia is shown in Fig 1. Further concrete explanations of each mechanism, based on the accumulated in-vitro and in-vivo data, are provided in the following sections. Of particular interest is the fact that M. pneumoniae can cause many kinds of vasculitic/thrombotic disorders. Mycoplasma pneumoniae may locally affect a vascular wall by inducing cytokines and chemokines such as tumor necrosis factor-┙ and interleukin-8, which cause local vasculitic and/or thrombotic vascular occlusion without systemic hypercoagulable state. Alternatively, generalized thrombotic vascular occlusion can occur as a result of a systemic hypercoagulable state which is in turn a consequence of immune modulation leading to the activation of chemical mediators such as complements and fibrin D-dimer. Although it is already well known that M. pneumoniae can cause a few coagulation abnormality disorders such as disseminated intravascular coagulation and stroke, M. pneumoniae remains under-recognized as a causative agent for many other vasculitic/thrombotic disorders involving various organs of the human body. One reason for this must be that the ability of M. pneumoniae to cause vasculitic/thrombotic vascular disorders through the local operation of chemical mediators such as cytokines in the absence of an apparent systemic hypercoagulable state is not yet widely known. In this chapter, the author presents organ-specific and systemic manifestations of vasculitic/thrombotic disorders that may be associated with M. pneumoniae infection.