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Microbe Evasion of Immune Responses

This module will help you

• see how microbes evade immune responses.
• understand the impact of HIV on the immune system
• test your knowledge and immunology problem-solving skills.

Microbe Evasion of Immune Responses
HIV and AIDS

Microbe Evasion of Immune Responses

In order to survive, a pathogen must be able to reproduce and to spread to other hosts. Microbes are selected for their ability to survive and produce many daughter cells while evading destruction by the host's immune system. Infectious organisms have evolved many ways of evading the immune system; the table below summarizes some of these microbial strategies.

Pathogens can evade the innate immune system by avoiding phagocytosis. Capsules, M protein, and fibrin coats do not bind the adhesion molecules used by macrophages and neutrophils to phagocytose pathogens. Other pathogens avoid destruction in the phagosome by inhibiting fusion with the lysosome and drop in pH or by leaving the phagosome. Toxoplasma gondii forms its own vesicles, so that its proteins cannot be processed efficiently by either the endosomal or cytoplasmic pathways. Other pathogens avoid complement-mediated lysis by blocking insertion of the MAC into their cell walls.

Pathogens have also evolved strategies to avoid adaptive immunity. Some cover their cell walls with host proteins. Many pathogens employ antigenic variation, in which many antigenic variants of a pathogen occur and immunity to one variant is not protective against another. Two very successful examples of antigenic variation are influenza virus and HIV. Both are RNA viruses whose error-prone RNA-copying enzymes result in antigenically distinct mutants. Influenza undergoes small changes each year (antigenic drift), and the changing serotypes require new vaccines. Occasionally two influenza viruses from different species co-infect an animal, usually a pig or a fowl, and exchange RNA; when this happens, large antigenic shifts occur which can result in pandemics of influenza like the one in 1918 that killed millions of people worldwide. The parasite Trypanosome cruzii undergoes programmed antigen variation. As the host makes antibodies to one surface antigen, it is quickly replaced by expression of a different one.

Exogenous pathogens block antibody function by making surface molecules that bind IgG Fc regions to inhibit antibody effector functions. Neisseria species produce IgA proteases that cleave IgA. Other pathogens block complement activation.

Endogenous pathogens avoid antibody and complement by remaining inside host cells. Viruses can remain invisible to the T cell by becoming latent in the host cell, not replicating and therefore not expressing viral proteins that can be processed and presented on Class I MHC. Other viruses avoid CTL by infecting nerve tissue in the CNS that express little Class I MHC. Viruses also block antigen processing and presentation by interfering with proteasome or TAP function, or interfere with the expression of membrane adhesion molecules required for T cell activation.

Staphylococcus aureus enterotoxin and TSST-1 (Toxic Shock Syndrome Toxin) disrupt immune system function by acting as superantigens. Superantigens bind outside the antigen-binding site of some Vb chains and outside the antigen-combining sites of some Class II MHC molecules. In this way they signal CD4 T cell activation in the absence of specific antigen binding. Many more T cells will have a particular Vb than will bind a specific peptide, because one Vb can be paired with several Va to form TCR with different specificities. Superantigens stimulate up to 10% of T cells to respond where antigen would normally stimulate only 0.001-0.01% of T cells to respond. The T cells respond by secreting cytokines that suppress immune responses. Superantigen also induces apoptosis in the superantigen-binding CD4 T cells, so T cells that can respond to the pathogen are deleted.

Pathogens can also express immunosuppressive cytokines; an example is an IL-10 analog made by Epstein Barr virus that suppresses NK and CTL activation. Mycobacterium leprae induces a non-protective Th2 response in some individuals; the protective Th1 response is inhibited by the Th2 cytokines. Measles virus suppresses both T and B cell responses for several months following the virus infection. The mechanism is unknown; measles infection of dendritic cells may in some way induce suppressor T cells.

Immune responses to some pathogens actually cause much of the pathogenesis associated with infectious disease. Inflammation due to Th1 responses to Mycobacteria causes more tissue damage than does the bacterium. Eggs deposited by Schistosome worms in the hepatic portal vein can become lodged in small vessels in the liver and induce activation of Th1 cells; the resulting inflammation damages the liver. The immune response to Respiratory Syncytial Virus (RSV) results in wheezing and bronchiole constriction. The mechanism was discovered when a killed virus vaccine to RSV was administered to infants. Vaccinated infants made a Th2 response to the killed virus; when they next encountered virus, IL-3, IL-4 and IL-5 produced by memory Th2 cells stimulated mucus secretion, construction of bronchiole smooth muscle, and influx of eosinophils. Vaccinated infants became more seriously ill from RSV than non-vaccinated infants. Dengue virus causes "breakbone fever", so named because of the intense pain associated with the disease. A second attack of Dengue is associated with internal hemorrhage and even more serious disease. A vaccine to Dengue virus was removed from use when it predisposed immunized children to the hemorrhagic form of the disease.

One of the most interesting examples of host-pathogen interaction is the case of Mouse Mammary Tumor Virus (MMTV). MMTV is a retrovirus whose genome must be integrated with that of the host for virus to be produced. MMTV is acquired by newborn mice from milk carrying virus from their mother's infected mammary gland cells. B cells in the intestinal epithelium of the pups are infected; however, the B cells must divide to produce virions. Infected B cells express a MMTV superantigen on their membranes which stimulates T cells to express CD40L; in response to the CD40L, B cells are activated to divide and produce virus. Virions infect mammary cells and are transmitted to the next generation by nursing females. In some mice, parts of the virus genome have been lost; these mice have endogenous MMTV sequences that encode superantigen without being able to produce virions. Superantigen is expressed in all cells, including APC in the thymus, so developing T cells which have the Vb that bind superantigen undergo apoptosis and are not present in the mature T cell population. The result of this clonal deletion is that mice bearing that endogenous MMTV genome will not produce virions in response to MMTV infection because no T cells can be activated to stimulate B cell proliferation. Several different MMTV superantigens exist and different mouse strains carry different endogenous superantigen sequences, so that no one mouse strain is susceptible to all MMTV infections.

HIV and AIDS

AIDS (Acquired Immune Deficiency Syndrome) is caused by the Human Immunodeficiency Virus (HIV), a retrovirus. ("HIV virus" is redundant!!) Two strains of HIV have been identified, HIV-2 in West Africa and India and HIV-1 throughout the rest of the world. It is estimated that more than 34 million people are infected with HIV and over 16 million have died from AIDS since the disease was identified 20 years ago. In Zimbabwe and Botswana, over 25% of adults are infected. Genetic evidence indicates that HIV spread to humans from other primates.

HIV is transmitted through transfer of body fluids or infected cells. The virus is present in infected CD4 T cells, macrophages, and dendritic cells and as free virus in blood, semen, vaginal fluid, and milk. It can be transmitted by heterosexual or homosexual contact, sharing needles, contact with contaminated blood or blood products, and to infants during pregnancy, vaginal birth, or breast feeding. Transmission depends on how much virus is present. Transmission rates to the fetus during pregnancy can be as high as 25%, but can be blocked by AZT treatment of the mother and delivery by Caesarian section. Transmission in breast milk occurs in as many as 40% of mothers who are newly infected but in fewer mothers who have antibodies to HIV.

A typical infection with HIV results in a transient viremia and drop in blood CD4 T cell counts. About 50% of newly infected people experience flu-like symptoms; others are asymptomatic. Once infected, humans experience an asymptomatic clinical latency for 2-15 years, during which HIV is produced and removed by the immune system and CD4 T cells are killed and replaced. Beginning during the initial HIV infection, most infected people produce anti-HIV antibodies (become seropositive). Lymph node follicular dendritic cells trap HIV and infect nearby T cells; numbers of infected circulating CD4 cells and free virions are low. The hematopoietic system replaces destroyed T cells, keeping CD4 T cell counts in the normal range (800-1200/ml of blood). Later in infection, replicating virus disrupts the follicular dendritic cells; more infected T cells appear in the circulation and damaged cell replacement declines. Lymph node architecture is damaged and virus can no longer be trapped, so circulating levels of free virus increase.

Eventually circulating CD4 T cell levels fall to less than 500/ml and opportunistic infections occur; at CD4 counts of less than 200/ml the infected individual has AIDS. Oral candidiasis and recurrent Mycobacterium tuberculosis are common during the early symptomatic phase of AIDS. Later, patients develop shingles (recurrent Varicella zoster - chickenpox - virus), Pneumocystis pneumoniae fungal pneumonia, B cell lymphomas, and Kaposi's sarcoma, an endothelial tumor caused by infection with Human Herpes Virus 8 (HHV-8) and cytokine stimulation. Cytomegalovirus and Mycobacterium avium complex infections are common in the late stages of HIV. Other common infectious agents seen in AIDS patients include the parasites Cryptosporidium, Toxoplasmosis, and Leishmania, viruses Herpes simplex I and II, fungi Cryptococcus neoformans, Histoplasma capsulatum, and Coccidioides immitis (valley fever), and the bacterium Salmonella.

HIV is a retrovirus, belonging to the Lentivirus family of viruses that cause slowly progressing diseases. Its closest relative is Human T Cell Leukemia Virus (HTLV) that was discovered at about the same time as HIV. HIV has an envelope, a protein capsid, and an RNA genome present in two copies. Cell tropism depends on host cell CD4 and a chemokine receptor used as a co-receptor for infection, either CCR5 or CXCR4. HIV gp120 attaches to CD4 molecules on Th cells or macrophages, and gp41 promotes fusion of HIV and host cell membranes in the presence of host cell chemokine receptor. HIV that is most readily transmitted sexually is macrophage-tropic, preferentially infecting macrophages expressing low levels of CD4 and the CCR5 co-receptor. People with a rare mutation in their CCR5 genes are resistant to HIV infection. Later in infection, a T cell tropic HIV is produced and infects Th cells expressing high levels of CD4 and the CXCR4 chemokine receptor.

Once in the host cell cytoplasm, HIV reverse transcriptase (RT) makes a double-stranded DNA copy of its genome, which integrates into host cell DNA. Macrophages produce HIV inefficiently, but activation of infected Th cells during an immune response induces transcription factor NFkB, which binds virus LTR to enhance transcription of HIV DNA and initiate vigorous virus replication in CD4 T cells. HIV kills some T cells directly by infection and cytopathic effects, and by blocking T cell activation through gp120 blocking surface CD4 and down-regulation of IL-2, IL-2R, and TCR expression. HIV binding to CD4 in the absence of other signals induces apoptosis, and membrane fusion of T cells expressing viral gp120 and CD4 results in formation of syncytia (fused multinucleate T cells) and cell death. NK cells and HIV-specific CTL lyse infected CD4 cells. There is also some evidence that HIV may act as a superantigen and deplete certain Vb TCR subsets. Loss of CD4 T cells depresses immune function.

The immune system responds vigorously to HIV with humoral and cellular immunity without fully eliminating the virus. Estimates from studies of infected people treated with protease inhibitors are that 10⁹-10¹⁰ virions are produced daily, with a serum half-life of about 6 hours. Productively infected naïve T cells have a half-life of about 2 days, while productively infected memory T cells have a half-life of about 2 weeks. It was hoped that protease inhibitor treatment that reduced detectable virion levels to background would eventually allow the body to eliminate HIV as latently infected cells died; to date it appears that latently infected cells persist after years of treatment.

Antigenic variation is thought to be the primary cause of HIV persistence in infected people. Both HIV reverse transcriptase and the RNA polymerase that transcribes new HIV genomes are error-prone enzymes, with a combined mutation rate of 3 x 10^-5/nucleotide/cycle. At this rate, many variants that escape memory cell recognition are produced over the course of the infection. Virus removal is due primarily to opsonizing antibody and complement; however, opsonizing antibody bound to HIV binds FcR-positive cells and allows HIV to infect them. Cellular immunity kills infected CD4 cells to eliminate virus, but CD4 cells are key to making immune responses. Loss of immune function makes the body susceptible to opportunistic infection from normally nonpathogenic organisms.

CD4 cells are produced to replace those lost to infection. Their source is not known; the thymus of post-adolescents produces few naïve T cells. They may come from replication of mature CD4 T cells. Macrophages that are infected by HIV without being killed produce cytokines thought to be responsible for the wasting seen late in AIDS.

Some people are rapid progressors who develop AIDS in only a few years following infection. Slow progressors remain asymptomatic for over a decade following infection. A very small number of people are nonprogressors, resistant to either HIV infection or progression to AIDS. The best studied resistance mechanism is lack of the CCR5 chemokine receptor required for HIV to infect macrophages. This mutation was discovered in people who were repeatedly exposed to HIV but showed no sign of infection or seroconversion. People homozygous for the CCR5 mutation (found in 1% of Caucasians but not in Japanese or West African subjects) were nearly completely protected from infection. (Interestingly, their immune responses were normal in spite of the absence of CCR5, illustrating the redundant nature of chemokine action). The few homozygotes who were infected are believed to have been infected by the CXCR4-tropic form of the virus. Heterozygotes for the CCR5 co-receptor showed a slower progression to AIDS. Other populations of long term nonprogressors who are HIV-positive (have antibodies to HIV) and of highly exposed but seronegative people who have HIV-specific CTL and Th1 cells are being studied.

Presence of HIV-specific antibodies is used for initial diagnosis. The culture-grown virus preparation used for the ELISA contains tissue culture proteins to which some people have antibodies, for example people recently vaccinated against influenza, so false positives can occur. A positive ELISA is followed by an confirmatory Western Blot to specific HIV proteins. Viral load is tracked using RT-PCR (see ToolBox); persistence of virus following initial viremia is predictive for rapid progression. Circulating CD4 T cell counts obtained with flow cytometry are also followed; high CD4 counts are associated with resistance to opportunistic infections.

HIV-infected people are treated with a cocktail of anti-viral drugs. Current medications include protease inhibitors that block virus packaging in infected cells, RT inhibitors, and drugs that block the integrase required for virus DNA integration into host cell DNA. Although the enzymes targeted are virus-specific, host cell enzymes are inhibited somewhat by the medications and side effects can be severe. Medications must be taken often and on a strict schedule during the day. High virus mutation rates result in drug resistance; giving several medications at once slows but does not prevent the advent of resistance. Medications were once given upon diagnosis, but more recently have been reserved for symptomatic patients to delay viral resistance. Drug costs are high and prevent most infected people in developing countries from being treated. Drugs which block infection by blocking CCR5 are being investigated.

Vaccines against HIV are in human trials, but the problem of inducing protective immunity is a formidable one. The rapid antigenic variability of HIV eludes the immune response in infected people and makes choosing antigenic variants for inclusion in a vaccine difficult. In addition, it is not clear what immune response is protective. If active CTL are needed to control the virus, infectious virus must be included in the vaccine, a real safety consideration. Recombinant and DNA vaccines containing only some HIV genes are more likely to be used than attenuated HIV (see Vaccines). Sterilizing immunity in which no infection occurs has so far been impossible to achieve with any vaccine. Latent persistence of infectious HIV makes virus elimination difficult even in immune people. Finally, it is unethical to vaccinate without doing all one can to prevent or treat infection, but vaccine efficacy is much harder to determine in populations where incidence of HIV infection is low or viral titers are reduced through use of anti-viral drugs.

Practice Quiz

Pick the one BEST answer for each question by clicking on the letter of the correct choice.

1. Pathogens evade immune detection by

a. altering their antigens periodically.
b. binding the Fc region of IgG to block complement activation and opsonization.
c. degrading IgA directed against them
d. living in the cytoplasm of host cells.
e. All of the above are ways pathogens evade immunity.

2. Which has NO effect on immune function?

a. Age.
b. Body temperature.
c. Nutrition.
d. Steroid hormones.
e. All of the above affect immune function.

3. Superantigens

a. avoid immune elimination by only activating T cells.
b. fit into the antigen-combining site of most TCR.
c. induce a very strong (super) immune response against the pathogens that produce them.
d. induce CD4 T cells to produce cytokines that inhibit an immune response.
e. suppress immunity by inducing clonal proliferation.

4. A virus that can remain latent in a cell avoids elimination by

a. ADCC.
b. CTL.
c. macrophages.
d. neutralizing antibodies.
e. opsonizing antibodies.

5. Staphylococcus aureus protein A, that binds the Fc region of Ig g chains, allows the bacterium to avoid elimination by

a. ADCC.
b. antibody opsonization.
c. classical complement activation.
d. antibody-mediated phagocytosis.
e. All of the above.

6. A virus could avoid CTL recognition by inhibiting the expression of

a. b2 microglobulin.
b. chaperones.
c. proteasomal enzymes.
d. TAP.
e. All of the above.

7. Mycobacterium leprae that induce a Th2 response avoid immune elimination by

a. activated macrophages.
b. ADCC.
c. antibodies.
d. CTL.
e. NK cells.

8. Tissue damage caused by the immune system responding to pathogens is usually due to

a. cytotoxicity by CTL.
b. cytotoxicity by NK cells.
c. inflammation.
d. memory cells.
e. superantigens.

9. Expression of endogenous MMTV superantigen on host cells

a. activates host cells to produce virus.
b. deletes host T helper cells specific for superantigen.
c. eliminates the host immune response to MMTV.
d. both b and c are correct.
e. a, b, and c are correct.

10. HIV can kill CD4 T cells by

a. diverting all their protein synthesis into virus production.
b. expressing gp120 on T cell membranes to make them targets for ADCC.
c. inducing apoptosis by binding to CD4 without being presented on Class II MHC.
d. inducing their lysis by HIV-specific CTL
e. All of the above probably occur.

11. A person has AIDS when s/he

a. becomes infected with HIV.
b. dies from HIV.
c. has fewer than 200 CD4 T cells/ml of blood.
d. has more than 200 HIV particles/ml of blood.
e. is seropositive for HIV.

12. Clinical latency means that

a. a balance exists between virus production and elimination.
b. anti-viral medications are still effective against HIV.
c. HIV is latent in host cells.
d. no disease symptoms are experienced.
e. no free virions are detectable in the blood.

13. People with AIDS

a. are always highly infectious.
b. die from opportunistic infections.
c. live a normal life-span if they take their medications on schedule.
d. rarely suffer side effects from the anti-viral medications.
e. would be expected to live on average another 10 years.

14. Current medications used to treat HIV infection interfere with all of the following EXCEPT

a. CCR5.
b. integrase.
c. opportunistic infections.
d. protease.
e. reverse transcriptase.

15. All of the following contribute to the difficulty in developing an HIV vaccine EXCEPT the

a. antigenic variation of the virus.
b. difficulty in inducing sterilizing immunity.
c. ethical considerations that require prevention and disease treatment during any vaccine trial.
d. need to induce opsonizing antibodies for protective immunity.
e. potential danger of a vaccine to HIV.

Problem

1. For each pathogen in the table above, describe immune responses that are still available to eliminate it.

2. Brainstorm the development of an HIV vaccine. How might you deal with some of the potential difficulties.

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