Lack of α-1 integrin alters lesion morphology during pulmonary Mycobacterium tuberculosis infection

Summary

The hallmark of Mycobacterium tuberculosis infection is the granuloma, a highly dynamic immune structure that contains the bacilli during chronic infection. Here, we examined if α1β1 integrin is required in the development and maintenance of the granulomatous structure during pulmonary infection using the α1 integrin knockout (α1-null) mouse. The α1β1 integrin is expressed on activated macrophages and T cells, and interacts with collagen molecules in the extracellular matrix (ECM), and thus may play a role in the granulomatous process. Following pulmonary infection with virulent M. tuberculosis, lungs of α1-null infected mice had striking differences in granuloma structure, as well as distinct and markedly thickened alveolar septae. By day 180, there were regions of cell death within granulomatous lesions, characterized by cellular debris in these mice. To determine if this molecule was necessary for T cell trafficking within the lungs, the expression of CD4, CD44 and CD62L was monitored. The number of activated and IFN-γ-producing CD4⁺ T cells increased in the lungs of α1-null mice during the chronic phase of infection, although they had decreased concentrations of TNF-α and MMP-9. These results suggest that while α1β1 integrin is not required for trafficking or maintenance of T cells in M. tuberculosis infected lungs, it does play a role in granuloma structure and integrity during the chronic phase of infection.

Introduction

Declared as a global health emergency in the early 1990s, tuberculosis (TB) remains a major health problem today and is one of the leading causes of morbidity and mortality worldwide, particularly in settings of poverty.¹ The World Health Organization (WHO) estimates that approximately 1.7 million people die annually from TB, and in 2004, approximately 8.9 million people developed the disease.¹ The emergence of multidrug-resistant TB (MDR TB) and, more recently, extensively drug-resistant TB (XDR TB), highlights the urgency of understanding and preventing this ancient disease.²

The hallmark of Mycobacterium tuberculosis infection is the granuloma, a highly dynamic immune structure that is thought to contain the bacilli during the latent period of infection. Granulomas are composed of multiple immune cells, mainly monocytes and lymphocytes, which are recruited to the site of infection through a complex network of cytokines and chemokines.3, 4, 5 Macrophages infected with M. tuberculosis release inflammatory cytokines interleukin-1 (IL-1), IL-6, IL-12 and tumor necrosis factor alpha (TNF-α).5, 6, 7 Acquired immunity to M. tuberculosis is mediated by IFN-γ-producing T cells, which activate infected macrophages to eliminate the bacilli, or at least contain mycobacterial growth.8, 9, 10 In humans, upon reactivation of the bacterium, necrosis can be followed by caseation, which in turn leads to a break down in the granuloma structure and bacteria are released into the airways for transmission to other hosts.¹¹

Integrins belong to a large family of transmembrane, heterodimeric adhesion receptors consisting of α and β chains and are involved in cell trafficking.¹² They can also serve as primary sensors of the extracellular matrix (ECM) environment, which initiate signaling pathways that regulate cell migration, growth and survival.13, 14 The α1β1 integrin, also known as “very late antigen” (VLA)-1, is a major receptor for collagen molecules on the ECM.¹⁵ The α1 integrin is expressed on, amongst other cell types, T cells, natural killer (NK) cells, NK T cells, and macrophages, and thus may be expected to play a significant role in mediating cell adhesion during inflammatory responses. In particular, α1β1 integrin has been shown to be expressed on activated T cells¹⁶ and on activated macrophages¹⁷ and associated with inflammatory diseases such as arthritis, sarcoidosis and asthma. While thus far few studies have been reported to show that α1β1 integrin plays a role in tuberculosis, it is up-regulated on T cells in humans during tuberculosis infection,¹⁸ and several studies have implicated the involvement of other integrins such as VLA-4 in the recruitment of leukocytes into infected lungs in a mouse model of tuberculosis.¹⁹

Matrix metalloproteinases (MMPs) play a major role in degrading the ECM and tissue remodeling during inflammation,²⁰ thus it may be anticipated that MMPs might be involved in regulating the interaction of activated T cells and macrophages during the inflammatory process. The interaction of the α1β1 integrin with the ECM down-regulates the expression of MMPs.²¹ Furthermore, α1 subunit knockout mice have increased MMP-9 activity, suggesting a down-regulatory role for this molecule.²²

In order to gain further understanding of granuloma development and progression and the role that α1β1 integrin may play in the process we employed a mouse model of pulmonary tuberculosis, using an α1 integrin (α1-null) mouse strain. Pulmonary infection with M. tuberculosis is known to cause recruitment of immune leukocytes leading to the formation of a granulomatous lesion. Given the fact that α1 integrin is utilized by leukocytes to bind to the ECM, we hypothesized that it is required by T cells and macrophages for maintenance of granuloma integrity.