1. Viral Tropism and Cellular Receptors

2. Viral Tropism
A specific pathologic “signature” left by a viral infection, usually related to its ability to replicate only in specific cell types
Samllpox (scarring from recovered scabes)
Polio (paralytic attack)
Yellow Fever (acute jaundice)
Rhinovirus (common cold)
Mechanisms of tropism
Cellular receptors; differential expression limits viral entry into specific cell types that express the cellular receptor
Post-entry factors; cell type must be permissive for viral replication
Transcription and translation machinery compatible with viral life cycle
Innate anti-viral defenses can be overcomed by viral encoded proteins or these defenses do not affect viral life cycle

3. For lipid enveloped viruses, a specific viral envelope
Attachment Protein (usually glycosylated) interacts with
the cognate cell surfcae receptor to bring about
fusion of the virus membrane and the host-cell membrane

4. Varieties of Viral Receptors

5. Attachment and entry are distinct events, and may be mediated by different receptors

6. Varieties of Viral Receptors
For some viruses, attachment and entry receptors are complementary but distinct
Herpes Simplex Virus 1 (“cold sores”)
Herpes Simplex Virus 2 (genital lesions)
Cellular attachment is primarily via heparan sulfate, but productive entry into the cell requires a coreceptor and interaction with a different envelope glycoprotein
Herpes Virus has 11 membrane glycoproteins, four of which are involved in attachment and entry
gB, gC (attachment--interacts with heparan sulfate)
gD, gH/gL (entry--interacts with coreceptor)
Expression of these receptors in the right place and combination can account for some of HSV tropism
Many coreceptors have been described

8. Entry receptors for Herpes Simplex Viruses

9. Varieties of Viral Receptors
For some viruses, attachment and entry receptors are complementary but distinct
HIV-1 (only one envelope glycoprotein)
Requires a primary receptor (CD4) and a coreceptor (chemokine receptor) for viral entry
Sequential interaction of the virus envelope glycoprotein with CD4 and coreceptor results in fusion between the virus and host cell membrane

10. Use of Different Coreceptors Largely Accounts for Viral Tropism (HIV)

11. Use of Different Coreceptors Largely Accounts for Viral Tropism (HIV)

12. This makes CCR5 an attractive drug target
ccr5/ccr5
81% of Caucasian
Get infected normally
Progress to AIDS normally
ccr5/Dccr5
15-18% of Caucasians
Get infected normally
But progress to AIDS 2-4 years
more slowly
Dccr5 /Dccr5
1% of Caucasians
Highly Resistant to Infection
Loss of CCR5 function but
otherwise normal (no side effects)
This makes CCR5 an attractive drug target

13. Varieties of Viral Receptors
For some viruses, attachment and entry receptors are the same, but membrane fusion is not at the cell surface, membrane fusion is triggered by low pH in endosomes
Influenza
HA glycoprotein binds to sialic acid receptor on cell surface
Virus and receptor are endocytosed
Low pH in endosomes trigger conformational changes in HA which results in membrane fusion

15. Fusion Peptide

16. Viral Receptors: Some Principles
A variety of molecules, including glycoproteins, glycolipids, and glycosaminoglycans, can serve as viral receptors.
Different viruses employ different cellular receptors.
A given virus isolate may employ several alternate cellular molecules as receptors.
In some instances, viral entry requires two or more different co-receptors on the cell surface. Usually, both co-receptors are necessary and neither alone is sufficient.
Different isolates of the same virus may prefer different receptors. A specific virus isolate may alter its receptor preference by selection of a mutant VAP during serial passage in animals or cell cultures.
The domain of the receptor that binds the virus may be either a polypeptide sequence or a carbohydrate moiety, often located at the external tip of the receptor molecule.
Not all cells that express the viral receptor are capable of supporting the complete cycle of viral replication.

17. Viral Tropism & Pathogenesis
Viral receptor expression is the primary but not the only determinant of viral tropism
Polio virus
Paralysis
Parvovirus B19
Exanthem subitum
Pure red cell aplasia
LCMV

18. Poliovirus Family:Picornaviridae (small (+) RNAvirus); Genus: Enterovirus
Infects only primates (human & non-human)
Virus bind more avidly to homogenates from primate vs non-primate tissues
Viral RNA infectious for single round in non-primate tissue (by passing the entry block)
PVR--CD155 (Ig Superfamily)
High conservation amongst Primates
Replicates in gut, excreted in feces
Fecal-oral transmission; hygiene issues; swimming pools
Viral invasion of CNS; also replicates preferentially in anterior horn cells--lower motor neurons of spinal cord; results in flaccid paralysis

19. Receptor expression is necessary but not sufficient to explain viral tropism
Polivirus Receptor Expression and Viral RNA expression

20. Parvovirus B19 Family: Parvoviridae (ssDNA virus)
Tropism for red cell progenitors in bone marrow
Receptor is a glycoshingolipid on the erythrocyte P antigen--also present on cells of mesenchymal origin
Virus replicates only in actively dividing cells, not in terminally differentiated RBCs
Therefore, tropism is limited to actively dividing cells with high receptor expression
(erythrocyte precursors)

21. RBCs
Platelets
Macrophages
Granulocytes

22. Lymphocytic Choriomeningitis Virus Family: Arenaviridae (minus-strand RNA virus)
Different strains of LCMV exhibit different tropism
Armstrong strain--neurotropic
Clone13 strain--hepatotropic (liver) and spleen-tropic
Both strains use the same receptor
(a-dystroglycan), but differ by one amino acid in the viral envelope glycoprotein
One amino acid change confers high or low affinity binding of viral envelope to cellular receptor

23. Other determinants of viral tropism
Cellular protease requirement
Temperature of replication
Acid Lability
Transcriptional control

24. Cellular protease requirement
Virulent
(NCD Virus)
Avirulent
Fusion Incompetent
Fusion
Competent
Secreted protease
Fusion
Competent
Plasma membrane
protease
cleavage

25. Temperature of replication
Optimal temp. for rhinovirus (common cold) replication is 33º C
Restricts replication to respiratory epithelium (e.g. lining of the nose and throat)

26. Acid/Protease lability
Enteric (gut) viruses must be able to survive low pH (acidic) of stomach, high pH of intestine and actions of digestive enzymes
Some viruses exploit presence of digestive enzymes to “activate” viral envelope protein
Reovirus

27. Transcriptional Control --retroviruses
LTR
Transcription
(tat)

28. DNA Binding Motifs for various transcription factors

29. Pappilomavirus Replication in Epidermis
Mature virions
and viral shedding
Transcription of late
Structural genes,
virion assembly
Primary Infection,
Permissive for genome
replication, but transcription
of late structural genes
is blocked

30. Tropism, Viral Variation and Pathogenesis/Virulence
Avian Influenza Virus
H5N2 serotype circulates in domesic fowl
Virulent serotype occurred in domestic poultry (1983 in Penn., 1995 in Mexico)
Virulence is due to single amino acid change in envelope--deleted a glycosylation site close to Env cleavage site

31. Mutant Virus
--replicate to higher titers, in wider range of tissues
--mild disease becomes rapidly fatal
Fusion
Competent
Plasma membrane
protease
cleavage

32. HIV tropism and Pathogenesis
MACROPHAGE-TROPIC
SHIV SF162P
LPL DEPLETED
LYMPHOCYTE-TROPIC 7
SF33A 7
LN DEPLETED 6 6
VIREMIA (RNA COPIES PER ML) 5 5 4 4 3 3 6 12 18 24 30 36 42 48
2.0
2.0
1.5
1.5
CD4:CD8 RATIO
1.0
1.0
0.5
0.5
0.0
0.0 6 12 18 24 30 36 42 48
WEEKS AFTER INFECTION