Clearance and Organ Distribution ofMycobacterium tuberculosisLipoarabinomannan (LAM) in the Presence and Absence of LAM-Binding Immunoglobulin M

Abstract

Lipoarabinomannan (LAM) is a component of the mycobacterial surface which has been associated with a variety of deleterious effects on immune system function. Despite the importance of LAM to the pathogenesis of mycobacterial infection, there is no information available on its fate in vivo. In this study, we determined the pharmacokinetics and tissue distribution of exogenously administered LAM in mice. For measurements of serum and tissue LAM concentrations, we developed an enzyme-linked immunosorbent assay which used monoclonal antibodies of different isotypes to capture and detect LAM at concentrations of >0.4 g/ml. Intravenous administration of LAM to mice resulted in transient serum levels with organ deposition in the spleen and in the liver. Immunohistochemical studies localized LAM to the spleen marginal zone macrophages and, to a lesser degree, to liver macrophages. When LAM was administered to mice previously given a LAM-binding immunoglobulin M (IgM), LAM was very rapidly cleared from circulation. In those mice, deposition of LAM in the spleen was significantly reduced while LAM deposition in the liver increased. Administration of LAM-binding IgM resulted in significant levels of IgM to LAM in bile consistent with an increased hepatobiliary excretion of LAM in the presence of specific antibody. Bile, liver extracts, and bile salts were found to rapidly inactivate the immunoreactivity of LAM. The results indicate that serum clearance and organ deposition of LAM in mice are affected by the presence of LAM-binding antibody and suggest a mechanism by which antibody could modify the course of mycobacterial infection.Lipoarabinomannan (LAM) is a mycobacterial envelope glycolipid (3) that has been implicated in the virulence and pathogenesis of Mycobacterium tuberculosis infection. In vitro LAM has been shown to inhibit the proliferation of human T cells (20), scavenge oxygen free radicals, block the transcriptional activation of gamma interferon-inducible genes, inhibit protein kinase C activity (4), and alter cytokine expression by macrophages (2). LAM has also been implicated in fever induction, tissue necrosis (24), and lung cavity formation (5). Despite considerable evidence that LAM can function as an immunomodulator, there is no information available on the fate of LAM in vivo. Another unexplored question is the role, if any, of LAMbinding antibody in altering the fate and tissue distribution in vivo. LAM is immunogenic and elicits antibody responses during natural mycobacterial infection (14, 29). There is evidence that some antibodies to M. tuberculosis may contribute to host defense (9). In this regard, a serum immunoglobulin G (IgG) response to LAM has been associated with reduced likelihood of M. tuberculosis dissemination in children (8).The liver and the spleen are important organs in the clearance of microbial products, and the presence of specific antibody can have profound effects on antigen clearance in vivo. For example, Cryptococcus neoformans glucuronoxylomannan (GXM) is deposited primari...

Antigenic Evidence of Prevalence and Diversity ofMycobacterium tuberculosisArabinomannan

“…The paucity of staining of liver and spleen tissues may be a result of the lower numbers of CFU in these organs or the ability of these organs to efficiently clear the polysaccharides. In this regard, a previous study (10) demonstrated that the liver and the spleen are very active in the pharmacokinetics of LAM, with the hepatobiliary system having an important role in clearance.…”

“…However, in recent years several studies reported a beneficial effect of monoclonal antibodies (MAbs) directed to mycobacterial antigens on the course of infection (for a review, see reference 8). Two of these studies involved MAbs directed to mycobacterial polysaccharides (10,24). These observations suggest that polysaccharides can potentially be useful for eliciting protective immune responses against M. tuberculosis.…”

“…The slides were washed with distilled H 2 O and decolorized with acid-alcohol. Immunohistochemistry was performed as described previously (10). Briefly, the slides were incubated with 10 g of MAb 5c11 per ml in 2% BSA for 2 h, followed by incubation with GAM-AP IgM (Southern Biotechnology Associates, Inc.) diluted 1:200 in 2% BSA.…”

Towards a New Challenge in TB Control: Development of Antibody-Based Protection

“…This study strongly supports an indirect effect of certain antibodies on the course of mycobacterial infection, altering problably the pharmacokinetics of mycobacterial components and contributing to protection against TB [56].…”

Capsular Arabinomannans from Mycobacterium avium with Morphotype-specific Structural Differences but Identical Biological Activity

“…Both glucan and AM were expressed in vivo and in vitro (5,6). It has been shown by immunoelectron microscopy that the AM is localized in the capsule (7). Another modified AM possessing serological activity has been isolated from Mycobacterium smegmatis (8).…”