Pathology of Influenza - Lecture 9

Acute diffuse inflammation of larynx, trachea, and bronchi- Characterized by:
- Death of columnar ciliated cells, resulting in a single cell basal layer.
- Edema with neutrophils and antigen presenting cells (APCs), specifically macrophages.
- Viral pneumonia (more severe):
- Marked by increased blood flow to the lower lungs.
- Thickening of alveolar walls.
- Edema containing APCs.
- Capillary dilation and thrombosis.
- Potential for alveolar rupture.
Repair process:- Basal cells resume mitosis approximately 3-5 days after infection.
- Repair and destruction can occur concurrently.
- Complete regeneration of epithelial damage takes approximately 1 month.

Stimulation of TLR3 and TLR7 leads to production of pro-inflammatory cytokines, notably IFN-a.
Functions of IFN-a:- Promotes influx of neutrophils.
- Initiates molecular antiviral responses.

Protein Kinase R (PKR) and 2’-5’ Oligoadenylate Synthase (2’-5’OS) are activated by viral double-stranded RNA (dsRNA):- Products of 2’-5’OS lead to activation of RNaseL.
- PKR phosphorylates eIF2, inhibiting translation:
- Resulting in blocked translation initiation due to phosphorylated eIF2.
- RNaseL digests viral RNA.
IFN-induced antiviral responses: Enhance cellular defense against viral replication.

NS1 is implicated in virus pathogenesis:- Mutants lacking NS1 produce increased levels of IFN.
Mechanism of NS1: - Functions by binding to and inhibiting cellular transcription of interferon-responsive genes.
- Binds to and inhibits action of cleavage and polyadenylation specificity factor (CPSF), disrupting the formation of 3' ends of mRNAs, hence affecting cellular gene expression.
Highly Pathogenic Avian Influenza (HPAI) H5N1 possesses an effective NS1 protein.

The adaptive response to influenza can be characterized by non-specific reactions:- Key Players:
- APCs (Antigen Presenting Cells): Dendritic cells, macrophages, B-cells.
- T-helper cells: Signal for expansion of immune response, promoting B-cell and Cytotoxic T Lymphocyte (CTL) proliferation.
- CTLs: Recognize and kill infected cells directly.
- B-cells: Produce species-specific antibodies, differentiating into memory and plasma cells.
Function of APCs:- Clear dying cells and present antigens to T-helper cells.
Recognition mechanisms:- CTLs identify infected cells through T-cell receptor/CD8 recognizing MHC1 + antigen interaction.
- B-cells recognize antigens via surface antibodies and present these to T-helper cells, leading to B-cell activation and differentiation into plasma cells.
Antibody types in response to influenza:- HA (Hemagglutinin) antibodies: Can be neutralizing (prevent infections).
- NA (Neuraminidase) antibodies: Non-neutralizing but inhibit the function of NA.
- CTLs target proteins including HA, NA, M1, NP, PB2.

Distribution of viral receptors impacts tropism and disease severity:- 2,6 linked sialic acid is present in the upper respiratory tract.
- 2,3 linked sialic acid is found in the lower respiratory tract.
Comparison of flu strains:- Seasonal influenza viruses bind primarily to 2,6 linked sialic acid.
- HPAI H5N1 binds to 2,3 linked sialic acid;
- Results in severe lower respiratory tract disease upon infection.

Understanding Receptor Specificity:- Alters the potential tropism of the virus to the lower respiratory tract.
Characteristics of HA:- Comprised of HA0, HA1, and HA2 cleavage sites.
- Cleavage of HA is essential for infection; fusion cannot occur without it:
- Cleavage generally happens post-release by a limited number of host enzymes in the respiratory tract, thus restricting tropism.
Influence of pH on HA conformational state:- At pH 7, HA is less effective in fusogenic activity.
- At pH 5, conditions favor HA fusion.
Poly-basic Run in HPAIs:- HPAIs, such as H5N1, have a poly-basic run of amino acids (lycines and arginines), allowing cleavage by widespread proteases (e.g., furins, PC6).
- If H5N1 breaches the respiratory tract (resulting from significant tissue damage), it can infect other tissues, leading to systemic organ failure, particularly in the kidneys.

Differences in pathogenesis between uncomplicated flu and viral pneumonia.
Role of the innate immune response in influenza infection.
Function of NS1 in influenza pathogenesis.
The contribution of the adaptive immune response in influenza infections.
Identifying significant influenza antigens in the adaptive response.
Explanation of how discrepancies in host receptors can lead to variations in tropism and disease severity in influenza.
Mechanisms behind HA cleavage and its relevance to pathogenesis and tissue tropism.

Protein Kinase R (PKR): Inhibits viral protein translation by phosphorylating eIF2, a key factor in translation initiation.
RNaseL: Activated by products of 2

-5

-Oligoadenylate Synthase (2

-5

-OS), it degrades viral RNA.

NS1: A viral protein that suppresses the host's interferon response by inhibiting the transcription of interferon-responsive genes and disrupting mRNA processing via Cleavage and Polyadenylation Specificity Factor (CPSF) inhibition.
Hemagglutinin (HA) Antibodies: Prevent viral infection by neutralizing the virus.
Neuraminidase (NA) Antibodies: Non-neutralizing but inhibit the neuraminidase enzyme's function, which is crucial for viral release.
Cytotoxic T Lymphocyte (CTL) Target Proteins (HA, NA, M1, NP, PB2): Key viral proteins targeted by CTLs for immune clearance.
Hemagglutinin (HA): A viral surface glycoprotein essential for binding to host cell receptors and facilitating membrane fusion; its cleavage and receptor binding specificity determine viral tropism and infectivity.