The Mnn2 Mannosyltransferase Family Modulates Mannoprotein Fibril Length, Immune Recognition and Virulence of Candida albicans
Immune Recognition and Virulence of Candida albicans. PLoS Pathog 9(4): e1003276. doi:10.1371/journal.ppat.1003276
The Mnn2 Mannosyltransferase Family Modulates Mannoprotein Fibril Length, Immune Recognition and Virulence of Candida albicans
Rebecca A. Hall 0
Steven Bates 0
Megan D. Lenardon 0
Donna M. MacCallum 0
Jeanette Wagener 0
Douglas W. Lowman 0
Michael D. Kruppa 0
David L. Williams 0
Frank C. Odds 0
Alistair J. P. Brown 0
Neil A. R. Gow 0
Marta Feldmesser, Albert Einstein College of Medicine, United States of America
0 1 Aberdeen Fungal Group, Institute of Medical Sciences, University of Aberdeen , Foresterhill, Aberdeen , United Kingdom , 2 Biosciences , College of Life and Environmental Sciences, University of Exeter , Exeter , United Kingdom , 3 Quillen College of Medicine, East Tennessee State University , Johnson City , Tennessee, United States of America , 4 AppRidge International , LLC , Telford, Tennessee , United States of America
The fungal cell wall is the first point of interaction between an invading fungal pathogen and the host immune system. The outer layer of the cell wall is comprised of GPI anchored proteins, which are post-translationally modified by both N- and Olinked glycans. These glycans are important pathogen associated molecular patterns (PAMPs) recognised by the innate immune system. Glycan synthesis is mediated by a series of glycosyl transferases, located in the endoplasmic reticulum and Golgi apparatus. Mnn2 is responsible for the addition of the initial a1,2-mannose residue onto the a1,6-mannose backbone, forming the N-mannan outer chain branches. In Candida albicans, the MNN2 gene family is comprised of six members (MNN2, MNN21, MNN22, MNN23, MNN24 and MNN26). Using a series of single, double, triple, quintuple and sextuple mutants, we show, for the first time, that addition of a1,2-mannose is required for stabilisation of the a1,6-mannose backbone and hence regulates mannan fibril length. Sequential deletion of members of the MNN2 gene family resulted in the synthesis of lower molecular weight, less complex and more uniform N-glycans, with the sextuple mutant displaying only un-substituted a1,6-mannose. TEM images confirmed that the sextuple mutant was completely devoid of the outer mannan fibril layer, while deletion of two MNN2 orthologues resulted in short mannan fibrils. These changes in cell wall architecture correlated with decreased proinflammatory cytokine induction from monocytes and a decrease in fungal virulence in two animal models. Therefore, a1,2-mannose of N-mannan is important for both immune recognition and virulence of C. albicans.
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Funding: This work was funded by the Wellcome Trust (080088, 086827, 075470 and 099215), and by a FP7-2007-2013 grant agreement
(HEALTH-F2-2010260338ALLFUN). MDL was supported by an MRC New Investigator Award (MR/J008230/1). The funders had no role in study design, data collection and analysis,
decision to publish, or preparation of the manuscript.
Competing Interests: I have read the journals policy and have the following conflicts: Douglas W. Lowman is an employee of AppRidge International, LLC. The
noted receipt of funding from AppRidge does not alter our adherence to all PLoS Pathogens policies on sharing data and materials.
Candida albicans is a polymorphic fungus that forms part of the
natural human microflora. However, many adverse conditions
result in predisposition to oral and vaginal infections and, under
circumstances where the host immune system becomes severely
compromised as a consequence of malignancy, trauma or
chemotherapy, C. albicans can invade underlying epithelial cells
and disseminate via the bloodstream and cause systemic disease.
The associated mortality rate of systemic disease is
approximately 3040%, which is higher than that observed for many
bacterial systemic infections, making C. albicans a major
pathogen of the immunocompromised and a significant global
health burden [13].
The fungal cell wall is a highly dynamic structural organelle
essential for maintaining cell shape and for protection against the
environment. The cell wall is also the first point of contact between
the fungus and the host, and as a result the cell wall is pivotal for
fungus-host interactions and immune recognition. The cell wall is
comprised of an inner skeletal layer of chitin and b-glucans
(b1,3and b1,6-glucan) and an outer layer of highly glycosylated
mannoproteins [46]. These proteins are decorated with linear
O-linked mannan and highly branched N-linked mannan, which
can be elaborated with additional mannan side chains attached via
a phosphodiester linkage known as phosphomannan (PM). The
mannan fraction of the cell wall is important for adhesion, cell wall
integrity, immune recognition and comprises up to 40% of the cell
wall dry weight [712]. All of the major cell wall carbohydrate
components of fungal walls serve as pathogen associated molecular
patterns (PAMPs), which are recognised by the innate immu (...truncated)