why herpes simple 1 remain latent according to Lange

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I now have all the key content from Jawetz (a Lange series textbook). Here is the complete, well-sourced answer:

Why HSV-1 Remains Latent - According to Jawetz, Melnick & Adelberg's Medical Microbiology (Lange Series)

1. The Route to Latency

After primary infection (usually oropharyngeal in childhood), HSV-1 replicates first at the mucosal site of entry. The virus then invades local nerve endings and is transported by retrograde axonal flow to dorsal root ganglia - specifically, oropharyngeal HSV-1 infection results in latent infection in the trigeminal ganglia.

"Virus then invades local nerve endings and is transported by retrograde axonal flow to dorsal root ganglia, where, after further replication, latency is established."

Jawetz, p. 483

2. The Nature of the Latent State

Once in the ganglia, HSV-1 enters a non-replicating state in which:

Virus resides in latently infected ganglia in a non-replicating state
Only a very few viral genes are expressed (the viral genome is not completely silent, but gene expression is severely restricted)
Viral persistence in latently infected ganglia lasts for the lifetime of the host
No infectious virus can be recovered from the usual sites of recurrence between episodes

"Virus resides in latently infected ganglia in a nonreplicating state; only a very few viral genes are expressed. Viral persistence in latently infected ganglia lasts for the lifetime of the host."

Jawetz, p. 483-484

3. Why It Remains Latent Despite Immunity

The latent state persists even in the presence of fully functional immune responses. According to Jawetz:

After primary infection, the virus is carried in a latent state in the presence of antibodies
These antibodies do not prevent reinfection or reactivation of latent virus - they only modify subsequent disease
Both humoral and cell-mediated immunity are present, but spontaneous reactivations occur despite HSV-specific humoral and cellular immunity in the host

This is because the virus hides inside neurons (which express very few viral antigens during latency), making it essentially invisible to the immune system.

4. Triggers That Break Latency (Reactivation)

Provocative stimuli that can reactivate the virus include:

Axonal injury
Fever
Physical or emotional stress
Exposure to ultraviolet light

When reactivated, the virus travels via axons back to the peripheral site (e.g., the lip), where replication proceeds at the skin/mucous membranes, producing the classic "cold sore."

5. Why Some People Reactivate and Others Don't

Jawetz explicitly notes: "More than 80% of the human population harbor HSV-1 in a latent form, but relatively few experience recurrences. It is not known why some individuals have reactivations and others do not." - p. 484

Summary Table

Feature	Detail
Site of latency	Trigeminal ganglia (HSV-1)
Viral state	Non-replicating; only very few genes expressed
Immune evasion	Hidden inside neurons; antibodies cannot prevent reactivation
Duration	Lifetime of the host
Reactivation triggers	Fever, stress, UV light, axonal injury

Source: Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Edition (Lange series), Chapter 33 - Herpes Simplex Viruses, pp. 482-486

Vp16 an LATs . action of those

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I have enough detail from the textbooks. The Jawetz replication cycle figure caption at line 498 actually describes VP16's function (as α-proteins/immediate-early gene products). Now I have comprehensive information from multiple sources. Let me compile the full answer:

VP16 and LATs in HSV-1 - Roles in Lytic Replication and Latency

VP16 (Virion Protein 16 / α-TIF / UL48)

VP16 is a tegument protein of HSV - it sits between the capsid and the envelope, so it is delivered directly into the host cell at the moment of viral entry, before any new viral gene transcription occurs.

Mechanism of Action

VP16 is a powerful transcriptional transactivator of immediate-early (IE / α) genes. Here is how it works step by step:

Entry: When HSV fuses with the plasma membrane, the viral capsid (with tegument) is released into the cytoplasm. VP16 dissociates and moves to the nucleus.
Complex formation: VP16 cannot bind DNA directly. It forms a tripartite complex with two host-cell proteins:
- Oct-1 (a cellular transcription factor that binds the TAATGARAT element in IE gene promoters)
- HCF-1 (Host Cell Factor-1, a chromatin-associated protein)
Transactivation: This VP16-Oct-1-HCF-1 complex binds to the TAATGARAT motif in the promoters of all five IE genes (ICP0, ICP4, ICP22, ICP27, ICP47), dramatically upregulating their transcription.

The IE Gene Cascade VP16 Triggers

As described in the Jawetz replication cycle figure (p. 482):

Phase	Proteins	Function
Immediate-Early (α)	ICP0, ICP4, ICP22, ICP27, ICP47	Activated by VP16; stimulate transcription of early genes
Early (β)	Thymidine kinase, DNA polymerase	DNA replication; produce concatemeric DNA
Late (γ)	Structural proteins, glycoproteins	Virion assembly, capsid, envelope

VP16 = the master switch that turns on the entire lytic replication cascade.
Without VP16 activity, lytic replication cannot be initiated.

"α-Proteins, products of immediate-early genes, stimulate transcription of early genes... β-Proteins, products of early genes, function in DNA replication... γ-Proteins, products of late genes and consisting primarily of viral structural proteins, participate in virion assembly."

Jawetz, p. 482

LATs (Latency-Associated Transcripts)

LATs are the only viral transcripts abundantly expressed during latency. They are the molecular counterbalance to VP16.

Structure

LATs are non-coding RNAs transcribed from a region of the HSV genome that is antisense to (overlapping but in the opposite direction from) the ICP0 gene
They exist as a stable 2 kb intron (and an 8.3 kb primary transcript) that accumulates in the nuclei of latently infected neurons
They are not translated into protein - they act as regulatory RNAs

Mechanisms of Action

1. Suppression of IE gene expression (anti-VP16 effect)

LATs encode multiple microRNAs (miRNAs) that act as antisense suppressors:

"Some of the microRNAs of LAT are antisense to ICP0 or α mRNA that encodes an immediate early protein necessary for lytic viral replication."

Sherris & Ryan's Medical Microbiology, p. 547

This directly blocks the IE genes that VP16 would otherwise activate, keeping the virus in a silenced state.

2. Repression of other viral lytic genes

"LAT encodes factors that repress expression of other viral genes, thereby preventing immune recognition of the viral proteins those genes encode."

Janeway's Immunobiology 10e, p. (chapter 13)

By silencing lytic genes, no viral antigens are produced on the neuron surface, making the infected cell invisible to cytotoxic CD8+ T cells.

3. Anti-apoptosis - protecting the host neuron

"LAT also interferes with apoptotic death of the host cell, thereby prolonging the cell's life span - and that of the viral genome it harbors."

Janeway's Immunobiology 10e

LATs keep the neuron alive for the lifetime of the host, providing a permanent reservoir.

4. Epigenetic silencing

The viral genome exists in the nucleus as a circular extrachromosomal episome (not integrated). LATs promote chromatinization and heterochromatinization of the lytic gene promoters, keeping them in a transcriptionally repressed state.

VP16 vs. LATs: The Lytic-Latency Balance

	VP16	LATs
Type	Tegument protein	Non-coding RNA / miRNA
Role	Drives lytic replication	Maintains latency
Target	IE gene promoters (TAATGARAT)	IE gene mRNAs (esp. ICP0)
Effect	Activates all lytic genes	Represses lytic gene expression
When active	During active infection / reactivation	During latency in neurons
Consequence	Productive viral replication, cell death	Silent persistence, neuron survival

The key interplay: During latency, LATs suppress IE gene expression including ICP0 - the very same genes that VP16 would activate if the virus were to reactivate. When reactivation stimuli (stress, UV, fever) overcome LAT-mediated suppression, VP16 can once again drive the IE gene cascade, initiating a new lytic cycle.

Sources:

Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Ed (Lange), Chapter 33, pp. 482-486
Sherris & Ryan's Medical Microbiology, 8th Ed, pp. 546-547
Janeway's Immunobiology, 10th Ed, Chapter 13

why multiple dose given in opv

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I have all the information I need from Park's and other sources. Let me now compile the comprehensive answer:

Why Multiple Doses of OPV Are Given

OPV is unique among live vaccines - while most live vaccines (measles, yellow fever, etc.) require only one dose for protection, polio vaccine requires three or more doses at spaced intervals. Park's Textbook of Preventive and Social Medicine explicitly states:

"The other exception is polio vaccine which needs three or more doses to be given at spaced intervals to produce effective immunity."

Here are all the reasons:

1. Interference Between the Three Serotypes (Intra-vaccine Competition)

OPV contains three live attenuated strains: Type 1, Type 2, and Type 3. When given together as trivalent (or bivalent) vaccine, the three strains compete with each other for replication in the intestinal mucosa.

In any single dose, one serotype tends to dominate and "crowd out" the others
The child may seroconvert to only 1 or 2 of the 3 types from a single dose
Multiple doses are needed to ensure seroconversion to all three serotypes

"Against a given serotype, monovalent OPV containing only that serotype is more immunogenic than trivalent vaccine because of a lack of interference from other serotypes."

Harrison's Principles of Internal Medicine, 22nd Ed

2. Interference from Other Enteric Viruses (Especially in Tropical Countries)

In tropical/developing countries, the gut is heavily colonized by many other enteric viruses (enteroviruses, reoviruses, etc.). These viruses compete for the same intestinal receptors and replication machinery, reducing OPV virus uptake.

Park's notes that even a single dose elicits substantial immunity - except in tropical countries - where intestinal interference from competing enteric organisms dramatically reduces vaccine take. This is a major reason children in developing countries need more doses than those in high-income settings.

3. Maternal (Passive) Antibodies

Newborns carry maternally transferred IgG antibodies against poliovirus. These passive antibodies:

Neutralize the live vaccine virus before it can replicate in the gut
Prevent establishment of intestinal infection needed to generate immune response
Wane over the first 6 months of life

Multiple doses ensure that at least one dose is given after maternal antibodies have sufficiently declined, allowing the vaccine to "take."

4. Need to Achieve Immunity to All Three Serotypes

A single dose may produce antibodies to only one or two serotypes. Since all three serotypes (Type 1, 2, 3) cause paralytic poliomyelitis:

Complete protection requires antibodies against all three types
Repeated doses, each with a chance of different serotypes "winning" the intestinal competition, ultimately ensure immunity to all three

5. Variable Immune Response / Vaccine Take Rate

OPV (given orally) depends on intestinal replication to stimulate immunity. The "take rate" (probability of successful infection and seroconversion) from a single dose is not 100%. Factors that reduce it include:

Diarrhoea at time of vaccination (the virus is washed away - importantly, Park's notes a dose given during diarrhoea should not be counted and should be repeated)
High ambient temperature degrading vaccine potency
Improper administration

Multiple doses compensate for these failures.

6. Immunological - Building Durable Mucosal + Systemic Immunity

OPV works by:

Infecting intestinal epithelial cells → replicates → spreads to Peyer's patches → secondary viraemia
Produces circulating IgG antibodies (prevents spread to nervous system)
Produces secretory IgA (prevents intestinal reinfection with wild strains)

Multiple doses strengthen and broaden this dual-layer immunity. OPV thus induces both local (mucosal) and systemic immunity, and repeated dosing reinforces both arms.

Summary Table

Reason	Mechanism
Inter-serotype competition	Types 1, 2, 3 compete in gut; one dominates per dose
Enteric virus interference	Other gut viruses compete for receptors (esp. tropics)
Maternal antibodies	Passively transferred IgG neutralizes vaccine virus
Need all 3 serotypes covered	Each dose gives chance to seroconvert to remaining types
Variable vaccine take	Diarrhoea, heat, improper admin reduce efficacy per dose
Strengthening dual immunity	Repeated dosing builds both mucosal IgA + systemic IgG

Source: Park's Textbook of Preventive and Social Medicine, Chapter on Poliomyelitis, pp. 244-246; Harrison's Principles of Internal Medicine, 22nd Ed, "Prevention and Eradication of Poliovirus"

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