Capsules and Their Functions in Immunology

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Questions and Answers

What are capsules?

Discrete extracellular components made up of polysaccharides which can be tightly associated or physically bound to bacteria's outer surface.

Which of the following are functions of capsular polysaccharides? (Select all that apply)

Cell division

Transmission (correct)

Adhesion (correct)

Evasion of immune response (correct)

How do capsules promote transmission?

By preventing desiccation of bacteria with a hydrate coating, allowing longer survival in external environments and increased chances of infecting a new host.

Give an example of capsule-mediated intracellular survival.

NMEC - K1 capsule prevents localisation of late endosomal markers in BMEC cells and prevents lysosomal fusion. Signup and view all the answers

What are some examples of capsule-mediated evasion of the adaptive immune response?

<ol> <li>Structural mimicry of host compounds; Neisseria meningitidis - capsule which mimics sialic acid. 2. Regulation of T-cell response; group B streptococcus has terminal sialic acid residues that can dampen the T-cell response.</li> </ol> Signup and view all the answers

What is an example of capsule-mediated evasion of the innate immune system?

Capsules are negatively charged, repelling PMNL interaction and can mask opsonins. Strepococcus pneumoniae recruits factor H, reducing phagocytosis. Signup and view all the answers

What is an example of capsule-mediated adhesion?

Group A streptococcus has hyaluronic acid capsule that binds to CD44 and facilitates efficient invasion of epithelial cells. Signup and view all the answers

How does capsule induce pro-inflammatory immuno-modulation?

Capsules from S.aureus bind epithelial cells, inducing the production of pro-inflammatory cytokines like IL-1 and IL-6. Signup and view all the answers

What is a mechanism for capsule anti-inflammatory immuno-modulation?

Bacteroides fragilis has polysaccharide A capsule which induces IL-10 production, while Salmonella enterica typhi’s Vi antigen capsule masks O-antigen. Signup and view all the answers

How do capsules affect fitness?

Capsules may or may not be required during an infection. Their expression or lack thereof can act as a population bottleneck due to variable efficacy. Signup and view all the answers

How are capsules used in disease treatment?

Conjugate capsule vaccines can generate immune memory against bacteria, such as the Hib vaccine, which was successful in generating herd immunity. Signup and view all the answers

Study Notes

Capsules Overview

Capsules are discrete extracellular components composed of polysaccharides, found either tightly associated with or physically bound to the outer surface of bacteria.

Functions of Capsular Polysaccharides

Facilitate transmission of bacteria.
Evasion of both innate and adaptive immune responses.
Host immuno-modulation to affect immune system response.
Promote adhesion to surfaces or host cells.
Enhance intracellular survival within host environments.

Transmission Promotion

Capsules prevent bacterial desiccation by coating them in a hydrated layer, allowing prolonged survival outside the host, thus increasing potential for transmission to new hosts.

Intracellular Survival Example

NMEC (Neonatal Meningitis Escherichia coli) K1 capsule prevents localization of late endosomal markers in brain microvascular endothelial cells (BMEC), thereby blocking lysosomal fusion.

Evading Adaptive Immune Response

Structural mimicry of host compounds; for instance, the capsule of Neisseria meningitidis mimics sialic acid, evading immune detection.
Regulation of T-cell responses; Group B Streptococcus has sialic acid residues on its capsule that bind to Siglecs on epithelial cells, dampening T-cell responses and reducing natural killer cell activity.

Evading Innate Immune System

Capsules exhibit a negative charge, repelling interactions with polymorphonuclear leukocytes (PMNLs).
Capsules can mask opsonins and facilitate improper deposition of the Membrane Attack Complex (MAC); for example, Streptococcus pneumoniae recruits factor H to cleave C3b opsonins, reducing phagocytosis.

Adhesion Examples

Group A Streptococcus utilizes a hyaluronic acid capsule to adhere to CD44, facilitating epithelial cell invasion.
Streptococcus pneumoniae capsule repels sialic acid in mucus, preventing entrapment and clearance by mucociliary action.

Pro-inflammatory Immuno-modulation

Capsules of Staphylococcus aureus (serotypes 5 and 8) bind to epithelial cells, promoting production of pro-inflammatory cytokines such as IL-1 and IL-6.
Streptococcus suis serotype 2 and Porphyromonas gingivalis K1 capsules induce production of MCP-1, enhancing inflammatory responses.

Anti-inflammatory Immuno-modulation

Bacteroides fragilis has a polysaccharide A capsule that stimulates IL-10 production in colon epithelial cells, promoting anti-inflammatory responses.
Salmonella enterica Typhi produces a Vi antigen capsule which masks O-antigen, inhibiting immune recognition and reducing production of pro-inflammatory cytokines like IL-17 and IL-8.

Capsules and Bacterial Fitness

The presence of capsules is not universally required throughout bacterial infections; some bacteria express capsules variably.
Diverse capsule serotypes contribute to varied functionality, influencing bacterial fitness and possibly acting as a population bottleneck due to expression variability.

Capsule Utilization in Disease Treatment

Conjugate capsule vaccines can generate immune memory to specific bacteria, exemplified by the successful Haemophilus influenzae B (Hib) vaccine, which has significantly increased herd immunity in the U.S. and Europe.
Immuno-modulatory effects from certain capsules can potentially be harnessed to mitigate inflammation during diseases like meningitis, where excessive inflammation can exacerbate outcomes.

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Description

This quiz explores the concept of capsules, particularly in microbiology, examining their functions, roles in transmission, and examples of their interaction with the immune system. Understand how capsular polysaccharides contribute to bacterial survival and evasion tactics against both innate and adaptive immunity.