BL 03-06-14 10-11am Immunology of AIDS - Cohen Flashcards
Acquired Immunodeficiency Syndrome defn.
Definition kept changing as learned more about its course & and causes
- Originally: ‘Any occurrence of an opportunistic infection or Kaposi’s sarcoma in pt w/out a previous Hx of, or apparent cause for, immune deficiency.’
- Now overall Dx made by detecting infection w/ HIV-1, the AIDS virus
test or by low Th (CD4+) cells
“Seropositive” to Dx HIV
Ppl are ‘seropositive’ if they have Ab to HIV
= most common way in which infection is 1st detected
Using Symptoms / CBC to Dx AIDs
HIV becomes AIDS once:
- pts get symptoms of opportunistic infections or Kaposi’s sarcoma
OR
- their ►Th (CD4+) cells fall below 200/μL of blood (normal range: 500-1000/ μL)
Cases of HIV in US in 2012
Total: ~35 million
Adults: ~31 million
Women: ~16 million
Children (<15yo): ~2 million
Cases of new infections w/ HIV in 2012
Total: 2.3 million
Adults: 2.2 million
Children (<15yo): 260,000
AIDS deaths in 2012
Total: 1.6 milllion
Adults: 1.5 million
Children (<15yo): 230,000
Per minute new HIV infections & deaths from AIDS
Every minutes:
- 4 ppl get HIV
- 4 die of AIDs
History of AIDS
- 1st described in 1981
- by 2004, 5th leading cause of death in the world, 4th in developing countries
Deaths from AIDS worldwide
- so far, 36 million have died worldwide
Cumulative cases of AIDS in US & number of deaths
- 1,160,000 cumulative cases of AIDS by end of 2012
- 800,000 had died
- 1,144,500 ppl living w/HIV in US, w/16% not knowing it
- 50,000 new cases each year
New treatments of AIDS results
- death rates have fallen remarkably
- from 38,000 in 1996 to 15,500 in 2010
OraQuick
= home test for HIV antibody, approved in May ‘12
- produces only 1:5000 false positives
- produces 1:12 false negatives
- –> so it’s useful but not definitive for reducing that 20%
AIDS in Colorado
- ~11,000 ppl living w/ HIV/AIDS in CO
- Incidence in CO has been falling since 1993
- in 2011, ~400 new cases & 150 deaths
Cause of AIDS
<— caused by virus called HIV-1 (Human Immunodeficiency Virus)
- HIV-2 has been isolated in West Africa
Human Immunodeficiency Virus (HIV)
A nontransforming retrovirus
= i.e., an RNA virus that carries no oncogene & reproduces itself by copying its RNA into DNA by means of its own enzyme, reverse transcriptase
- Similar to visna virus of sheep, equine infectious anemia virus, and feline immunodeficiency virus
—> all cause slow, ultimately fatal illnesses
Closest relation of HIV & Development of HIV
- HIV is most closely related to a Simian Immunodeficiency Virus, SIV
- It is thought that HIV-1 evolved recently from SIV, perhaps as recently as the 1940s & possibly in Zaire (now the Democratic Republic of Congo)
- We can try to determine when it originated by checking banks of stored human sera for Abs
- 1st sera in USA w/ Ab to HIV-1 are found in 1978
- 1st seen sera In Africa are from 1959
- HIV-1 sequences have been cloned from blood sample of 1959 from D.R. Congo
- Thus, seems relatively new
- Probably jumped from simian to human & not yet adapted to its new host (similar in that respect to Ebola and Marburg viruses)
Movement of HIV/AIDS
Seroepidemiology indicates that HIV moved:
- to the Caribbean in the mid-60’s (perhaps brought by Cuban soldiers returning from Angola)
- to Europe a bit later
Epidemic in USA started in New York, Los Angeles, & San Francisco
- probably brought in by men who had vacationed in Haiti
- By 2012, AIDS has been detected in every country in the world
Haitians & AIDS
Very high incidence of AIDS among recent Haitian immigrants & refugees
- made ppl think Haitians were particularly at risk
- now known to be untrue
- ‘risk groups’ now replaced by ‘risky behaviors’
Antigenic variability of HIV
HIV = most antigenically variable pathogenic virus yet encountered
- Reverse transcriptase is a highly error-prone enzyme, w/out proofreading capability
- –> makes mistake in ~1/100,000 base replications
- –> so infected ppl have many variants in their body
Risk groups vs. Risk behaviors
- “Risk groups” have now been replaced by “Risky Behaviors”
Risky behavior is whatever increases your chance of receiving an inoculum of HIV
= sexually transmitted. so frequent sex is risky if it involves partners who might have HIV
= Any lesion on or injury to mucous membranes increases risk.
= Injection of blood containing virus is highly risky
Drugs use is NOT thought of as risky, per se, nor is use of sexual stimulants like amyl nitrite.
Heterosexual contacts now account for more than half of new cases worldwide, & more than
half of those are women and girls.
Risk of getting HIV infection from injection of blood containing the virus
= highly risky, but much less so than w/ blood containing hepatitis virus
= In over 3000 reports of accidental exposures of US health care workers to HIV, only 9 were
documented to have become antibody-positive
Children w/ HIV
- Over 10,000 children have been reported as HIV+ in the USA
= mostly of mothers whose behaviors put them at high risk for acquiring HIV infection (drug abusers, sex workers)
Prognosis of AIDS
- 60% of all reported cases have died
- 95% of those Dxed before 1985 have died
- Original impression: most pts in whom AIDS was diagnosed would die of the disease, with mean survival time of ~3.5 years
- Now, w/treatment, much more optomistic than that
- New treatments have extended life expectancy remarkably.
- None of these treatments was available when the AIDS pandemic started.
HLA & AIDS
Link btwn HLA genotype & susceptibility:
- 65% of ‘elite controllers’ (harbor HIV but retain normal immune function for years) are HLA-B57+
- –> more, and more diverse, CTL against HIV peptides than people w/ other HLA alleles
another link between HLA-B57 and AIDS?
?
Time frame of pathogenesis of HIV
- After single exposure, infected people develop high blood virus levels peaking ~6 weeks.
- Ab to HIV peak by 9 weeks, whereupon virus levels fall sharply, but not to zero.
- Untreated seropositive people develop AIDS symptoms at the rate of ~3% per year.
- Mean incubation period is ~9.5 years w/out treatment.
1st cycle in Pathogenesis of HIV
Virus enters body & adheres to a lectin on DCs (DC-SIGN1)
—> allows virus to be taken up w/out being harmed (DC as Trojan horse to get virus to lymph nodes where Th are)
HIV binds by its envelope glycoprotein (gp120) to CD4 on Th cell surface
- –> induces conformational change in gp120
- –> allows it to bind co-receptor (chemokine receptors, either CCR5 or CXCR4)
- When 1st infected, almost all virus is CCR5-tropic
- Binding chemokine receptor changes conformation of gp41 glycoprotein associated w/ gp120
- –> exposes hydrophobic region that literally melts away T cell’s membrane
- –> cell & virus fuse
- –> virus injects its core into cell, activate its reverse transcriptase, & make dsDNA from its RNA
DNA moves into nucleus & integrates into random break in host DNA, helped by viral integrase
–> latent virus
HIV latency
- We know little about how latency is regulated, or whether it is harmless to the cell.
- HIV may go latent in resting cells & replicate productively in activated ones
- –> If true, this poses real problem for therapy: it may be bad to try to stimulate the immune system.
Encoding from HIV genome
By using all 3 reading frames, the small HIV genome (9749 bases) encodes 9 genes:
- gag, pol, & env genes that all retroviruses have,
- 6 others that regulate latency & virulence.
By alternative RNA splicing & protease-mediated cleavage of 3 large precursor proteins (HIV makes its own protease) it can make 16 polypeptides
CCR5 gene mutations
- ~10% of Caucasians have mutant allele of CCR5 gene (32-bp deletion)
- Ppl w/2 CCR5delta32 alleles do not express any surface CCR5
- –> they can be chronically infected w/HIV but do not become ill
- Seems probable that the infection in these ppl remains in DC & macs and does not affect Th cells
2nd cycle in Pathogenesis of HIV (inital coping then break down of immune system)
As viruses bud en masse from infected cell, tear so many holes in membrane that cell dies
In early, pre-AIDS stage, clearance rate of virus & replacement rate of CD4 cells are incredible:
—> ►estimated that entire population of virus is replaced daily & CD4 cells every 3 days
W/ time, CD4 cells gradually lost
= simple exhaustion of ability to produce them
—> commonly expressed as a falling blood CD4/CD8 ratio (normal ratio from ~1.5 to 3)
–> An accelerating fall in this ratio, or an absolute CD4 count below 400/microL, are poor prognostic signs
When immune system can no longer cope, opportunistic infections take hold.
Significant behavior of HIV virus
When virus is replicating, gp120/gp41 is made early, & inserts into cell’s plasma membrane
—> ►allows fusion of infected cell to nearby uninfected CD4 cells
—> syncytium forms
Thus, virus can spread w/out an extracellular phase
This could easily explain why the Ab pts make seems to be useless.
Overall progression of HIV/AIDS
- Acute infection
- Phase of clinical latency (infection w/out symptom) that may last years
- Development of minor opportunistic infection (OI) like Candida albicans (a yeast) of the mouth, esophagus, or rectum
- –> fevers, night sweats, weight loss & fatigue - Full-blown AIDS, with the development of:
- major opportunistic infections (including TB)
- malignancy (Kaposi sarcoma, or less commonly Burkitt or other lymphoma)
- absolute CD4 count <200the
Progression takes on average 9 years.
With treatment, no longer inevitable.
Syndromes w/ HIV
- Most common condition is to be seropositive w/out symptoms (~1,000,000 ppl in US)
- Minor opportunistic infection (OI) (Candida)
- Major opportunistic infections (TB)
- Malignancy (Kaposi sarcoma, Burkitt / other lymphoma)
- late AIDS dementia complex
Late AIDS dementia complex
- Brain also has cells that can be infected by HIV (including macs & microglia_
- Not-uncommon
- Terribly distressing for pts & family
<— Probably the consequence of toxic cytokine release by virus- activated phagocytes
Type of Secondary Infections in HIV/AIDS
Infections seen in AIDS are primarily of types that require T cell-mediated immunity (as might be expected given the virus’ primary target)
- Viral infections
- Fungal infections
- Protozoan infections
- Opportunistic intracellular bacteria
NOT high-grade extracellular bacterial pathogens
Viral infections seen in HIV/AIDS
- CMV
- Hepatitis
- esp. Herpes simplex & zoster
Fungal infections seen in HIV/AIDS
- Candida albicans
- Pneumocystis jirovecii
Protozoan infections seen in HIV/AIDS
- Toxoplasma
- Cryptosporidium (–> sometimes-fatal diarrhea)
- Isospora
Bacterial infections seen in HIV/AIDS
Infections w/ opportunistic intracellular bacteria:
- Mycobacterium avium complex (MAC)
- M. tuberculosis
- TB is the leading cause of death in people infected w/ HIV
- NOT infections w/ high-grade, extracellular bacterial pathogens
- less of a problem, possibly b/c ability to make T-independent Ab responses to capsular polysaccharides is preserved.
Kaposi’s sarcoma
= a tumor of the endothelial cells lining lymphatics
- more common in homosexual males w/ AIDS than in others (reason not known)
<— Caused by a newly emerging virus, called KSHV (Kaposi’s sarcoma herpesvirus) or HHV8 (human herpesvirus 8)
Diagnosis of HIV/AIDS
- Pt often has made the Dx
- Most common test is for Ab to HIV (ELISA + Western Blot)
- PCR Test for virus RNA
Detecting antibody to HIV to Dx
Antibody measured by ELISA
- has a certain false-positive rate, so a + ELISA must be confirmed by Western Blot analysis
Western Blot analysis: Standardized viral protein preps are: - separated by electrophoresis - blotted & fixed to nitrocellulose - ‘stained’ w/ pt's Abs ---> which must bind to correct viral proteins (gp120, gp41) for test to be considered a true positive.
PCR for Viral RNA to Dx & follow HIV
►Very small amounts of virus RNA itself
can now be detected by PCR
- Very useful for following therapy
- Mostly available in wealthier areas, which have a better success rate in AIDS treatment b/c they can do this sort of test.
- In patients who can be gotten down to ~50 viral particles/mL & kept there, disease progression seems to be halted.
Immune cells involved in HIV
Antibodies that pts make are not protective.
- They bind virus, but do not block attachment to & infection of Th cells.
- There are neutralizing epitopes on the virus, but they are shielded by carbohydrate & not readily available to B cells.
It seems that response to HIV is tilted towards Tfh & Th2 rather than Th1
- –> so the helping of CTL activation via IL-2 from Th1 is diminished
- ►If pts made more Th1 they might stimulate more CTL & do better
- Evidence that HIV+ pts who make good CTL against HIV antigens have improved prognosis.
Revolution of the Treatment of AIDS
- When AIDS was 1st IDed, there were no anti-retroviral drugs
- many ppl thought there never would be, b/c viruses use our human metabolic pathways to do their evil deeds
- –> anything that killed a virus would be very likely to kill us too.
Fortunately (one of the few good things to come out of the AIDS pandemic) they were wrong.
- Viruses may be parasites but they’re not Mini-Me, and they do have vulnerabilities.
Classes of HIV/AIDS Drugs
Reverse transcriptase (RT) inhibitors, 2 classes:
- Nucleosides
- Non-nucleoside inhibitors
- often both classes used together
Protease inhibitors for gag-pol-env polyprotein
Small peptide fusion inhibitor - Enfuvirtide (Fuzeon®)
Small-molecule CCR5 antagonist - Maraviroc (Selzentry®)
Integrase inhibitor - Raltegravir
Standard antiretroviral therapy (ART) = protease inhibitor + 2 reverse transcriptase inhibitors
The cost of caring for an AIDS patient exceeds $25,000/year in the USA, which is of course greater than the health budgets of most of the world’s villages. The ethical and practical problems surrounding trials and prices of new drugs, especially in the Third World, are formidable. However, several generic pharmaceutical companies around the world have defied USA and other patent laws and prepared cheap 3-drug combination ART pills that are available in sub-Saharan Africa for about $100-$150/year. Only about 9.7 million of the 35 million HIV+ people get them (but that’s up from 50,000 in 2002).
Reverse Transcriptase Inhibitor Drugs - Types
Reverse transcriptase (RT) is unique to retroviruses, using RNA template to create DNA, so it’s a target.
2 classes of RT inhibitors: Nucleoside & Non-Nucleoside Inhibitors
Nucleosides
= competitive inhibitors
= chain-terminators
Non-nucleoside inhibitors:
= bind hydrophobic pocket on enzyme
—> changes conformation & thus the activity of catalytic site
B/c escape from inhibition due to mutation is so common, using each of these classes of drugs together greatly lowers the odds of escape.
Protease inhibitors drugs for HIV/AIDS
The gag, pol, & env proteins are made as a single gag-pol-env polyprotein which the virus cleaves using its own protease
- This protease has thus become a drug target for protease inhibitors.
Enfuvirtide (Fuzeon®)
= a small peptide fusion inhibitor
- binds to part of gp41 so it cannot change conformation to fuse viral membrane w/ helper cell’s
Maraviroc (Selzentry®)
= a small-molecule CCR5 antagonist
- Binds to transmembrane portion of CCR5
- –> changes in conformation of external receptor
- –> no longer engages gp120
- –> blocks viral entry into CD4+ cells
Raltegravir
When viral DNA copy reaches the nucleus, a viral integrase function (part of RT complex) inserts it randomly into cell’s DNA.
- Raltegravir (FDA approved in 2007) blocks that step
- –> effective in patients w/ RT inhibitor-resistant strains of HIV
Standard antiretroviral therapy (ART)
= combines a protease inhibitor and two reverse
transcriptase inhibitors
Cost of caring for AIDS patient
Cost of caring for AIDS patient >$25,000/year in US
= greater than health budgets of most of the world’s village
- Ethical & practical problems surrounding trials & prices of new drugs, esp. in Third World, are formidable.
- BUT. several generic pharmaceutical companies around the world have defied USA & other patent laws and prepared cheap 3-drug combination ART pills that are available in sub-Saharan Africa for about $100-$150/year.
- Only ~9.7 million of the 35 million HIV+ people get them (but that’s up from 50,000 in 2002).
Prevention of HIV/AIDS in lay ppl
= Safe sex, safe addictions
- You DO NOT get AIDS from casual contacts
- Male circumcision is very effective & a growing practice in parts of Africa.
- Condoms work if they are used & stay intact.
- Spermicides DO NOT work
- Anti-HIV drug (tenofovir) in barrier gel has partial effectiveness
- Prophylactic ART protects non-infected member of a couple
- ART therapy to pregnant HIV+ mothers protects the fetus.
Prevention of HIV/AIDS in health professionals / lab workers
- Common disinfectants (alcohol, Clorox) kill HIV readily.
- Use hepatitis precautions.
- If do flow cytometry on human blood, be sure to understand production of aerosols by the machine
Vaccine problems & prospects
- Only a vaccine will ever be able to put an end to
this terrible worldwide epidemic. - In practice, frustrating to try
- Need a vaccine that can preferentially stimulate Th1 cells & CTL
- Current vaccines seem to be best at inducing Ab responses (doesn’t protect very well)
- Key epitope on HIV seems to be well-concealed within the gp120/gp41 complex & almost invisible to B cells.
- However, shown that small amount of Ab some people make is broadly neutralizing (bnAb)
—> analysis of the recognized epitopes could lead to a new ‘designer’ vaccine.
HIV vaccine challenges
- HIV exhibits tremendous global genetic diversity.
- Immense mutational capacity allows evasion of both T & B cell immunity.
- HIV goes latent in host genome, from which it cannot be eliminated by conventional antiretroviral drugs.
- No known example of spontaneous immune clearance, to use as basis for data-driven vaccine design.
- Though bnAbs found, they are rare, only seen in a subgroup, take years to develop, & extensively hypermutated; no method exists now for induction of these Abs by immunization.
