Lecture 31. HIV-AIDS. Dr. Jeffery DeStefano
AIDS (Acquired Immune Deficiency Syndrome)
Epidemiology- 683,000 cases in U.S. from 81-1998 and several million in Africa. Estimate about 1 million in
U.S. are infected and about 20 million in Africa. About 500,000 deaths from AIDS in US 1981 to 2003.
Approximately 30 million HIV infected worldwide as of 2003. In the U.S. it is estimated that 1/100 males and
1/800 females are infected. Worldwide, about 5 million people were infected with HIV in 2003. Current death
rate from AIDS in the US is about 10,000-15,000/yr (about the same as an average flu year). This is down from
over 50,000/yr. in the early and mid 90s. The main reason for the decrease is better drug therapy and to some
extent education. Recent estimates suggest that worldwide as many as 3 million people died of AIDS in 2003.
About 500,000 of those were children under the age of 15.
Other statistics USA:
-78% of AIDS cases in men are between ages 20-44.
-Males about 80%, females about 20%, worldwide females 40% and growing, about two female infections per
minute in the world or over 1 million per year.
-About 1.5% of total AIDS cases are children born to HIV positive mothers.
-Men: 53% white, 30% black, 17% Hispanic
-Woman: 55% black, 24% white, 21% Hispanic
-73 cases per 100,000 black woman and 5 per 100,000 white
-2 per 1000 college students are HIV-infected
Facts about the virus:
-an “infectious dose” (a dose which is likely to establish an infection, even much smaller doses could sometimes
cause an infection) of HIV may be 10,000 or more particles.
-In a person with AIDS there are about 4.2 million HIV particles per ml as compared with 100 million to 1
billion in a hepatitis B infection. The risk of getting Hep B from a needle stick (contaminated needle) is 12 to
60 times greater than getting HIV from a stick which is less than 0.5%.
Show table on page 149 (7-5) indicating the amount of HIV particles in different fluids and cells. Note that it is
quite low. However, the levels in the cells of certain fluids is relatively high. In Semen for example, 0.01-5%
of the T4 and macrophages are infected. This calculates to about 100-10,000 infectious units (plaque forming
units) per ml. However, the level of HIV that is cell-free in semen and most other fluids is quite low. Some, if
not most HIV infections are probably established by cells passing on virus directly to other cells via syncitia
formation or just adhesion (sticking together). In fact, T4 lymphocytes can pass HIV to cervical epithelial cells
where the virus can produce progeny. This likely occurs by the T4 cell adhering to the epithelial cells since cellfree HIV cannot infect epithelium. Infected cells can also be swept up by the lymph system where they will gain
access to many more T4 cells and macrophages.
Transmission-3 major modes:
-Listed is the proportion of each group as of 1994. A total of 442,000 cases had been reported in USA by 1994.
1. Sexual intercourse-accounts for about 60% of cases, higher risk to passive (receptive) partner. Anal
intercourse estimated at 1% chance per episode and vaginal about 0.1%. Risk increases 10-fold or more if
person is infected with other STD especially if genital ulcers are present (syphilis, Herpes). Number of partners
is an important factor.
2. Perinatal infection-about 1-2% of total infections. Baby has 12-40% chance of being infected if mother is
HIV positive and not treated by anti-HIV drugs. Several factors are involved including the stage and severity of
the maternal infection. Transmission can probably occur prenatally across the placenta, intrapartum during
parturition, and postnatal through breast milk. Current anti-HIV drug treatment of infected pregnant woman
significantly decreases the likelihood of the newborn becoming HIV positive.
1
3. Blood transfer:
transfusions-3-5% total. 90% chance of infection with infected blood. Since the blood supply is now
screened for HIV, the number of cases has decreased greatly. In 1994, this group was less than 1% of
the new cases reported in that year.
Injecting Drug users (IDUs)-25-30% of total cases generally from needle sharing. Risk may be
comparable to that for needle-stick injuries to medical workers (about 0.5% per episode).
HIV is also found in trace amounts from time to time in other bodily fluids including saliva, there is no
evidence for infection by casual contact like kissing.
Other minor contributors are things like needle sticks in health care workers.
Pathogenesis
-Depletion of CD4+ lymphocytes is the hallmark of AIDS. CD4+ T cells (T helper cells) are the major reservoir
and macrophages are the second largest. Reports of infection of CD4 minus cells like neurons and epithelial
indicate that HIV may be able to use another receptor to enter cells or cell to cell contact is used. It is likely that
HIV can initially enter the system (among other routes like direct entry into the lymph system) via passage
through epithelial cells. The virus may be pasted to the cells by direct contact with infected T cells or
macrophages that are present in semen or vaginal secretions of infected individuals. There is no evidence that
HIV replicates in the epithelial cells. Replication can occur in macrophages or T cells. In the former case HIV
buds on the ER membrane where it acquires its envelope and is then transferred by vesicles to the cell
membrane where the vesicles fuse and release the virus. This is similar to the way coronaviruses replicate.
Replication in T cells results in budding from the plasma membrane. Note that only activated T cells will allow
replication of the virus although the virus can persist in unactivated cells until they are activated by various
cytokines often secreted in response to infections with other organisms (like other viruses). After initial
infection in genital or rectal area the virus is transported to lymph nodes where it replicates extensively. Many
infected individual experience a brief glandular fever-like illness 1-3 weeks after infection which is associated
with a high virus titer in the blood and a decline in CD4+ T cells. A vigorous immune response ensues and
within a month the viremia (virus in the blood) declines to near undetectable levels. CD8+ cytotoxic T cells,
Natural Killer (NK) cells, and antibody-dependent cell-mediated cytotoxicity (ADCC), and neutralizing
antibodies may all contribute by lysing infected cells and destroying free virus. After seroconversion
(production of antibodies and cellular immunity in the blood stream, complete at generally 1-3 months after
infection) there is a long “asymptomatic” period of 1-15 years (10 average) with lower levels of CD4+ T cell
PBLs (peripheral blood lymphocytes which are lymphocytes (immune cells like CD4+ T cells and others) in the
blood stream) being infected although higher levels of infected cells are present in lymph nodes. However, the
level in the blood stream is a good predictor of the overall level in the body. Even during this “latent” period
the infected host has an increased risk of certain diseases (see handouts). A slow progressive decline in CD4+ T
helper cells is observed. The depletion of T helpers probably occurs by several mechanisms including direct
cytotoxicity, syncytia formation, immune cytolysis due to specific immune cells (like CTLs) recognizing viral
antigens presented on the infected cells surface, and apoptosis which is a cell suicide mechanism normally
involved in self-tolerance. As the level of CD4+ cells drops to between 200-400 cells/l (normal level is 9001200) a variety of opportunistic infections with various microorganisms begin to occur (Show Fig. 35.9 from
Medical Virology 4th, White and Fenner). A given individual may only show a few of the diseases. Among the
most common infections is pneumonia associated with Pneumocystis carinii and this is a common cause of
death. Several new drugs which can prolong the life of AIDS patients are being used in the treatment of
opportunistic infections (Show Figure from Stine 1996). Note that people infected with HIV do not necessarily
have AIDS. The definition of AIDS is complex. An individual can be defined as having AIDS if they have a
certain specific diseases that are only observed in people with AIDS. Or if they have other diseases usually
2
associated with AIDS and are positive for infection with HIV (by the standard HIV blood test which looks for
antibodies against HIV in the blood or looks directly for HIV proteins or by other PCR-based tests that look for
the viral genome in the person). Finally those individuals with a T4 helper cell count of less than 200 cells/l or
total T4 helper cells that are less than 14% of total lymphocytes who are also HIV positive are defined as having
AIDS.
What is happening in an HIV-infected person during the latent period?
It was once believed that there are many latently infected cells in these people. That is, cells infected by HIV
that are not active in producing virus but can be activated by various signals at some time. They would
represent a reservoir for the virus. Since they are not producing HIV they would not be detected by the immune
system. Hence it would be very difficult to eliminate HIV from a person. Current models suggest that only a
very small percentage of infected cells are in a latent state, perhaps 1% or less. Most are productively infected.
During the asymptomatic stage of infection a low, but steady-state level of virus is detected (show Figure from
handout). How is this steady-state maintained? Is it via a low level of virus production balanced by immune
system killing or a high level of virus production balanced by the immune system? To determine this we must
disturb the steady-state and see what happens. This was done by using drugs that does not affect existing
viruses but prevents the production of new progeny. The drug must be “essentially” 100% effective in
preventing new virus production. As it ends up, essentially any of the current HIV drugs will work in this test
(AZT was originally used). We can analyze the rate at which the viral population declines to estimate the
survival time of average viruses and the rate at which T4 cells recover can tell us how fast T cell are being
produced:
Drug treatments and what they have taught us:
-100-fold reduction in virus within 2 weeks after treatment with essentially any one drug of the classes of HIV
inhibiting drugs discussed in class (nucleoside RT inhibitors, nonnucleoside RT inhibitiors, protease inhibitors)
-Average lifetime of an infected cell is about 1-2 days
-5% of total T4 T cells are productively infected in an average individual at any given time during the latent
period. This is about 1010-1011 cells. At least 109-1010 cell deaths per day. Consistent with the estimate of at
least 109-1010 new viruses each day. Although this level of virus production is not particularly high relative to
other virus infections, keep in mind that it continues essentially every day for 10-15 years!
-Since the level of T4 cells does not change very much during the latent period the cells that are dying each day
are being replaced nearly as fast as they die. This means that bone marrow and spleen and other reservoirs for T
cells must be working at an accelerated rate.
- Within a few weeks to months after treatment with any one drug there is emergence of resistant virus. The
rapid emergence implies that the resistant virus was already present in the population at the time drug treatment
was initiated.
-The resistant virus reaches a slightly lower steady-state level than the original viruses (wild type) implying that
it does not grow quite as well as the wild-type.
-Upon removal of the drug, in the case of AZT (and essentially any of the single drugs), the wild type once again
becomes predominant over a long period of time. The relatively long period that it takes the wild type to regain
dominance suggests that the resistant mutant is only slightly less productive than the wild type (i.e. has a
slightly lower evolutionary “fitness”).
-Results suggest that one potential factor in the development of AIDS may be excessive stress on the immune
system due to rapid turnover of T cells that must be replenished.
What has triple drug therapy added?:
-Triple drug therapy requires that the resistant virus have many mutations such that it will be resistant to all
three drugs used. These mutants will likely be significantly less fit than the wild type so even if they do occur
3
the viral load will be significantly less.
-As it ends up the triple mutants are more fit than we would have hoped but still a lot less fit than wild type.
-Since the triple mutants are so complex, even with the large amount of replication and recombination and high
mutation rate or HIV these mutant may not be produced very frequently. Therefore, many infected people may
not have a specific triple mutant that can escape the therapy.
-This points out the importance of treating a person early since this give the virus less time to replicate and
produce mutants that could thwart the therapy.
-Thus far many patients have responded well to triple drug therapy and often no virus at all is detected after
treatment suggesting that the person does not have a mutant resistant to all 3 drugs being used. However, in
every case that a person has been taken off the therapy, wild type virus has re-emerged. This suggests that there
must be a reservoir of very long-lived cells that harbor the virus and can become activated to regenerate the
infection. It is not clear what these cells are (possible memory T cells) or how long they can live.
Example of viral population under different conditions:
-mutants are generally present at low levels in the viral population. Their levels depend on their relative
“fitness”. Mutants which are more fit will be present in greater proportions but the wild type, being the most
fits under normal conditions will dominate the population. A change of environment (drug treatment for
example) can effect the relative fitness of the viral population. A particular mutant could be more fit than the
wild type under specific conditions. In general, mutants do not replicate as well as wild type so even under
conditions where they have a selective advantage they will not reach the same level as the wild type can under
“untreated” conditions. The level which they do reach relative to the wild type level will depend on how well
the virus can replicate. Drug resistant mutant that replicate poorly compared to the wild type will, in the
presence of the drug, obtain steady-state levels well below that obtained by wild type in the absence of drugs. It
is possible that a particular multi-drug regiment could select for a viral mutant that replicates very poorly.
This might significantly reduce the burden on the patient's immune system and perhaps even prevent the onset
of AIDS. It is even possible that a particular regiment may select for a complex mutant not yet present in the
population. In this case perhaps the infection could be stopped. This emphasizes an important point: The
earlier that a HIV-positive person is treated with drugs the better the chance that the treatment will be
successful. Those infected for long periods will have had more time for the virus to generate mutants. Although
simple single base change point mutants are generated very rapidly, more complex mutants with several
particular changes within their genomes would take considerably longer to be produced. Early treatment may
stop the virus before such mutants are generated. Of course, since mutants are generated randomly, each
infected person will have a somewhat different set of
mutants, therefore a particular regiment, although
effective for some, may not be effective for all. Note
that a particular drug regiment may or may not
totally eliminate the wild type. This would depend on
the wild types' relative fitness in the presence of the
drug. The net result is that drugs don’t stimulate the
production of new viral progeny that are resistant to
the drug. They simply select for resistant viruses
already present in the population.
Why does HIV escape the immune response?
-Genetic variation- HIV-1 strains between infected
individuals can differ in nucleotide sequence by as
much as 20% and as much as 3% in a single
individual. Much variation is observed in the env
region which is consistent with alteration of the
4
antigens presented to the immune system. HIV is a "quasi-species" a collection of genomes related to but
significantly different from a particular master sequence. There are also specific changes in the pol gene
observed after treatment with specific anti-pol drugs. These variations are often specific for a certain drug.
The properties of the viruses found early in viral infection often differ from those found as the patient
progresses toward advanced disease with the later being more pathogenic to T cells and replicating more
rapidly. Why is there so much genetic variation?
-Inaccurate RT and no proof-reading activity.
-Recombination between genomes producing new progeny. Recombination occurs by a mechanism
similar to that described earlier for poliovirus.
-High number of replications per day in infected host (perhaps 1010 new viruses each day). So there is a
great chance for mutants to be generated.
-Other reasons for escape
-Latently infected cells can harbor virus can harbor virus without being susceptible to immune system.
These cells, though relatively low in number, may include some very long-lived immune cells. One
possibility is that they are some type of “memory” immune cells. These are the types of immune cells
that protect people for a long period of time against infection by specific pathogens. In a normal
immune response they are generated after the system first sees a pathogen (or vaccine). These cells then
serve as the basis for the more rigorous immune response that occurs on subsequent exposure to the
pathogen. The cells can potentially survive for many years and could serve as a reservoir to replenish
HIV infection.
-Cell to cell contact via syncytia and adhesion can spread infection with no virions to stimulate
immunity.
-In macrophages HIV buds from ER so macrophages have no viral proteins in the plasma membrane to
stimulate immune response.
-Since the virus infects and destroys cells of the immune system there is a progressive loss of immune
competence during infection.
Control of HIV infection:
1. Reduce number of Sex partners. Ideally mutual monogamy.
2. Use condoms but be aware that this is not 100% protective.
3. Treat all STDs promptly.
Clearly there are social and political ramifications related to the AIDS epidemic that are beyond the scope of
this class.
Therapies:
Vaccines are the ultimate goal in controlling AIDS (show table 35-5, Medical Virology 4th, White and Fenner).
So far vaccines have shown little promise although some gains have been observed.
Problems:
-genetic variation makes it difficult to produce a vaccine that is effective against all serotypes. Ultimately the
hope is to make a vaccine against a epitope that is universal (cross-reacts to all strains).
-HIV may enter body as an infected lymphocyte instead of a virion. The lymphocyte could pass the virus to
other cells by cell to cell contact initially avoiding the immune response. Once latent infections are established,
the virus would have time to mutate and possible escape the vaccine-induced immunity.
-Infected cells may be sequestered from the immune response in such sites as the Central nervous system.
Antiviral Drugs
Most are directed against the RT and these include among others AZT, ddI, and ddC, as well as the
nonnucleoside compounds like nevarapin. All the drugs can have side effects although the nucleoside analogs
5
tend to be more severe. These drugs have been helpful in prolonging the life of infected individuals but, when
used alone, mutant viruses rapidly develop that overcome the drug. Drugs directed against the viral protease
protein have also been developed. Saquinavar is among this group. Recent drug therapy has focused on using
multiple drugs simultaneously to treat patient. Triple drug therapy seems to be most effective. This generally
consists of 1-2 nucleoside inhibitors, 0-1 nonnucleoside RT inhibitors, and 1 protease inhibitor. If a particular
combination does not work due to toxicity or simply being ineffective in knocking down the viral load (could be
due to a mutant virus that can escape all 3 drugs and still replicates fairly well or to the drugs being metabolized
before they get into the blood stream) a different combination is tried until one works or it is deemed that the
person cannot be treated effectively (fortunately most can be). There are a number of new drugs for treatment of
the opportunistic infections associated with AIDS including sulfa drugs for pneumocystis and acylovir for
Herpes simplex and cytomegalovirus and interferon a for Kaposi’s sarcoma.
HIV Evolution
-Viral origin- Some antibodies of proteins of HIV cross-react with SIV proteins. The high degree of crossreactivity suggests that HIV evolved from SIV and was transferred to humans by monkeys. Transfer of virus to
human may have been around 1930-1940. First known AIDS case was 1959.
-HIV probably started as a relatively non virulent virus in humans. Social changes may have influenced the
rate of transmission of HIV and changed the way that natural selection affected traits of the virus. In an
isolated monogamous population HIV would be best relatively benign, leaving the host alive and mildly
infectious for many years. There is clear evidence for this. A cluster of people infected with HIV from a single
source in Australia have survived with little evidence of AIDS for several years. There are also monogamous
groups in Africa infected with a form of HIV-2 that shows little virulence.
-In a crowded population with expanded sexual networks, new host would be more available and infected hosts
more dispensable driving the virus to increased virulence. For example, a highly virulent strain may kill people
in 3 years instead of 30 but make them more infectious while living. This virus would still come out ahead in
evolutionary terms if there were several hosts available to infect.
-Taming HIV- One possibility is to push the virus toward more benevolent strains by depriving the virulent
strains of the high transmission rates they need to survive (condoms, clean needles, safe sex).
NOTES FROM POWERPOINT PRESENTATION
-HIV is a relatively minor STD by sheer numbers
-HIV is the only STD that causes substantial death totals worldwide although there are still several infectious
agents that cause more deaths, especially in developing countries. However, deaths from HIV to increase.
Estimated annual mortality approximately doubled over the last 10 years.
6