Deck 13: Failures of Host Defense Mechanisms

Question 1

Important information can be learned by studying the immune system of female carriers of the XLA disease gene. These individuals, identified as mothers of boys with XLA, show non-random X chromosome inactivation in their B cells, but random X chromosome inactivation in all of their other cells, including their T cells and macrophages. This finding indicates that:

A) The BTK protein is expressed by bone marrow stromal cells that support B cell development.
B) The BTK protein is required in hematopoietic stem cells.
C) The BTK protein is required for mature B cells to enter lymphoid follicles.
D) B cells and T cells do not share a common progenitor.
E) B cells require the BTK protein for normal development.

Answer 1

B cells require the BTK protein for normal development.

Question 2

Patients with X-linked lymphoproliferative syndrome type 1 (XLP1) have mutations in the SLAM-family adapter protein, SAP. These patients suffer from an inability to control infections of the herpesvirus Epstein-Barr virus (EBV) that infects B cells. Studies have been done using peripheral blood T cells from these patients to examine their response to EBV in comparison to the responses of normal individuals (EBV-infected) as controls. These data were obtained by ELISPOT analysis, to quantify the numbers of IFN- $\gamma$ -producing CD8 T cells per million peripheral blood leukocytes in response to several EBV peptides, indicated as peptide 1, 2, 3, or 4 (note: the patients and controls were matched for HLA class I molecules).

These results indicate that:

A) XLP patients have a defect in priming CD8 T cell responses following EBV infection.
B) XLP patients require IFN- $\gamma$ production to kill EBV-infected B cells.
C) XLP patients have normal CD8 effector cell priming following EBV infection.
D) XLP patients have a defect in cytotoxicity by NK cells rather than by CD8 T cells.
E) XLP patients have no defect in cytotoxicity by CD8 T cells.

Answer 2

XLP patients have normal CD8 effector cell priming following EBV infection.

Question 3

Bone marrow transplantation is currently used to treat many immunodeficiency diseases resulting from defects in hematopoietic cells, such as lymphocytes or myeloid cells. Often, the donor of the bone marrow is a healthy relative of the patient, such as a sibling or a parent .
Which of the individual relatives above would not be a potential donor for the patient's bone marrow transplant?

A) Mother
B) Father
C) Grandmother
D) Sibling 1
E) Sibling 2

Answer 3

Sibling 1

Question 4

Studies performed in mice have revealed one important component affecting the altered immune response in individuals undergoing malnutrition or starvation. In these studies, mice were placed on a starvation diet for several days, and then immunized with a protein antigen in an adjuvant. One group of starved mice were given injections of 'compound X' for 48 hours, starting at the time of immunization. Seven days later, draining lymph nodes were isolated, and T cell responses were measured in vitro following stimulation with antigen. The results are shown in Figure .
'Compound X' is most likely:

A) IL-1 $\beta$
B) Leptin
C) TNF- $\alpha$
D) IL-10
E) Double-stranded RNA

Question 5

Defects in components of the complement pathway do not lead to recurrent or persistent virus infections.

Question 6

Muckle-Wells syndrome is an autosomal dominantly inherited disease due to mutations in NLRP3. Individuals with this disease suffer from episodes of fever, as well as urticarial rash, joint pains, and conjunctivitis. What is the explanation for the beneficial effects of anakinra (IL-1 receptor antagonist) treatment in these patients?

Question 7

Infants with RS-SCID generally require treatment by bone marrow transplantation from a healthy donor. In this treatment, hematopoietic stem cells from the donor will give rise to normal B and T lymphocytes, thereby restoring normal immune function. However, the bone marrow transplantation treatment does not alter the high incidence of cancer in these patients. Why not?

Question 8

A variety of genetic defects can result in patients exhibiting SCID. For many of these diseases, infants with the disease are given bone marrow transplants from healthy donors to restore normal immune function. Yet, some diseases causing T cell deficiencies cannot be treated by bone marrow transplantation. Which of the diseases below is treatable by giving patients hematopoietic stem cells from a healthy donor?

A) XSCID
B) Di George syndrome
C) FOXN1 deficiency
D) MHC class II deficiency
E) MHC class I deficiency

Question 9

Individuals that lack all T cells have the most severe form of immunodeficiency (SCID) and will not survive past their first birthday without a bone marrow transplant from a healthy donor. These individuals fail to make antibody responses to the normal childhood vaccines because:

A) They generally lack all B cells
B) SCID causes a defect in B-cell receptor signaling
C) SCID patients lack AID, so their B cells cannot undergo class switching
D) Most antibody responses require T cell help for the B cells
E) Persistent infections in SCID infants disrupts hematopoiesis

Question 10

One group of immune deficiency diseases is caused by an inability of CD8 effector T cells to kill virus-infected target cells, due to defects in cytotoxic vesicle exocytosis. Because of the inflammatory response that accompanies a normal virus infection, together with the prolongation of this response due to the inability to control the infection, patients with these disorders suffer from tissue damage caused by the infiltration of effector CD8 cells and activated macrophages into multiple organs. In addition, a subset of these patients also show increased susceptibility to extracellular and intracellular bacterial infections. This is because:

A) CD8 T cells are required to kill extracellular bacteria.
B) CD8 T cells in these patients are defective in producing IFN- $\gamma$ .
C) Some proteins required for cytotoxic vesicle exocytosis are required for phagosome-lysosome fusion.
D) Inflammatory cytokines in these patients are inducing macrophages to phagocytose red and white blood cells.
E) Persistent uncontrolled herpesvirus infections cause immunosuppressive effects on bacterial clearance mechanisms.

Question 11

The current view in the field of immunology is that dendritic cells are the primary antigen-presenting cells for stimulating naive T cells. One piece of evidence supporting this conclusion is the observation that IRF8-deficient individuals, which retain their tissue-resident macrophages, are susceptible to a range of severe opportunistic infections caused by intracellular bacteria, viruses, and fungi. Explain the reasoning behind this argument.

Question 12

Patients with Wiskott-Aldrich syndrome show severely impaired responses to vaccines such as the tetanus vaccine, which is composed of the inactivated tetanus toxin (i.e., tetanus toxoid). Yet, these patients can generate normal antibody responses to bacterial polysaccharide antigens. This selective defect in antibody responses in Wiskott-Aldrich syndrome patients is due to:

A) Defective antigen presentation by dendritic cells
B) Impaired phagocytic function of macrophages and dendritic cells
C) Defective polarized secretion of cytokines by CD4 T cells
D) Impaired B cell activation to monovalent, rather than polyvalent antigens
E) Impaired migration of B cells to germinal centers

Question 13

Some forms of SCID are due to defects in common 'housekeeping' enzymes, such as enzymes involved in nucleotide biosynthesis pathways, that are present in all cells of the body. These genetic deficiencies cause SCID because:

A) The enzymes responsible are normally expressed more highly in lymphocytes than in other cells of the body.
B) The absence of these enzymes causes build-up of intermediates in the pathway that are toxic to developing lymphocytes.
C) The absence of these enzymes deprives lymphocytes of purine nucleotides that are needed for their development.
D) The absence of these enzymes impairs the extensive proliferation of developing lymphocytes needed to generate the large number of circulating B and T cells.
E) The enzymes involved in these nucleotide biosynthesis pathways cannot be compensated for by redundant pathways in the cells.

Question 14

Yersinia pestis, the causative agent of the bubonic plague, has multiple mechanisms of immune evasion. This Gram-negative bacterium is transmitted from fleas (body temperature, 26°C) to humans (body temperature, 37°C) by flea bites. Studies have shown that the lipopolysaccharide (LPS) produced by Y. pestis grown at 37°C is about 10-fold less potent at stimulating TLR4 signaling than is the Y. pestis LPS from bacteria grown at 26°C. When these two forms of Y. pestis LPS are compared for their abilities to induce responses from human macrophages, one would expect that the 26°C Y. pestis LPS would result in:

A) Increased production of antimicrobial peptides
B) Reduced production of IL-10
C) Increased production of TNF- $\alpha$ and IL-6
D) Reduced production of Type I interferons
E) Increased activation of the inflammasome

Question 15

Unlike defects in antibodies or T cell functions, defects in complement components often lead to autoimmune-like symptoms, rather than to increased susceptibility to infections. This is because:

A) Complement components do not require antibodies to attack microbial pathogens.
B) The complement pathway normally functions to clear immune complexes from the circulation.
C) Complement inhibitory proteins are up-regulated by complement activation.
D) The complement pathway is part of the innate, rather than the adaptive immune response.
E) Partial activation of the complement cascade promotes phagocytosis of host cells.

Question 16

Individuals with a complete absence of B cells and antibodies, such as patients with XLA, show a limited range of susceptibilities to infection rather than a global immunodeficiency to all categories of pathogens. For example, XLA patients show increased susceptibility to pyogenic bacterial infections, as antibody binding to these microbes is critical for their uptake and destruction by phagocytes. Clinicians caring for these patients are advised regarding their vaccinations, some of which could be highly dangerous to the antibody-deficient patient. In particular, XLA patients should never receive:

A) The tetanus toxoid vaccine composed of the inactivated toxin protein
B) The pneumococcal vaccine composed of polysaccharide antigens from 9 strains of Streptococcus pneumoniae
C) The influenza vaccine, composed of inactivated influenza virus
D) The Hib vaccine, composed of Haemophilus influenzae type b polysaccharides conjugated to the tetanus toxoid protein
E) The live oral polio vaccine, composed of an attenuated strain of the enteric poliovirus

Question 17

A mutant mouse line (Mutant-X) is discovered that is defective in generating IgG or IgA antibody responses to immunization with the inactivated influenza A virus vaccine. As a first step in determining the gene responsible for the immunodeficiency, T lymphocytes and B lymphocytes are separately isolated from wild-type or Mutant-X mice and mixtures of cells are transferred into Rag-deficient recipients, which are then immunized with the Influenza A vaccine as shown in Figure. Fourteen days later, serum from the immunized mice is collected, and tested for anti-influenza IgG and IgA antibody titers. The results are shown in Figure : mice is:

A) NEMO
B) CD40-ligand
C) CD40
D) AID
E) TACI

Question 18

Individuals with defects in ubiquitously expressed DNA repair proteins have a form of SCID known as RS-SCID (radiation-sensitive SCID). Studies have shown that, in addition to immune deficiencies, these patients have an increased rate of cancer. Yet, in general, they are diagnosed first based on their immunodeficiency disease, not on their susceptibility to getting cancer. This is due to the fact that:

A) The immune response to infections is required almost immediately after birth.
B) Cells other than lymphocytes have additional redundant DNA repair pathways.
C) More people have immunodeficiency diseases than have cancer.
D) Cancer only occurs in older individuals.
E) If treated for their immunodeficiency disease, the increased cancer incidence goes away.

Question 19

A mouse model of an immunodeficiency disease affecting the innate immune response was generated by knocking out a single gene. Studies of these mice, performed in the 1990s, showed that they were highly susceptible to infections of pyogenic bacteria, such as Staphylococcus and Lactobacillus species, and to infections of fungal pathogens, such as Paecilomyces. In fact, without deliberately infecting the mice, the researchers performing these studies found that the knockout mice were spontaneously succumbing to these infections while housed in their vivarium (animal facility), as shown in Figure.
To determine the potential mechanism leading to the immune deficiency, a series of additional studies were performed. In one study, peripheral blood leukocytes from the mice were mixed with Staphylococcus aureus bacteria, and the ability of the cells to kill the bacteria was assessed. In a second study, the compound thioglycolate was injected into the peritoneal cavity of the mice, and five hours later the numbers of white blood cells were measured. This compound elicits a robust inflammatory response in the peritoneum, including the production of several inflammatory cytokines, such as TNFα, IL-1, and IL-6, as well as a number of chemokines known to recruit neutrophils and monocytes from the blood to the site of inflammation. The results of these studies are shown in Figure .

Given these data, a likely candidate for the gene that was targeted in these knockout mice is:

A) IRAK4
B) CD18, the integrin $\beta$ chain
C) TLR3
D) G6PC3, the glucose-6-phosphatase catalytic subunit 3
E) p47, a component of NADPH oxidase

Question 20

Nearly all immune deficiency diseases that result in impaired T_H17 and ILC3 function lead to chronic mucocutaneous candidiasis (CMC), usually accompanied by increased susceptibility to pyogenic bacterial infections. However, a subset of these patients also show increased susceptibility to intracellular bacterial infections, such as those caused by Mycobacteria species. What is the explanation for why some, but not all, of these patients have increased susceptibility to intracellular bacteria and which patients fall into this category?

Question 21

A small group of HIV-infected individuals are known as 'elite controllers.' These individuals have HIV viral RNA copy numbers that persistently remain <50 copies/ml of plasma. Studies examining the immune response to the virus in these individuals would likely reveal:

A) Increased numbers of virus-specific CD4 and CD8 T cells secreting IFN- $\gamma$ compared to individuals with high viral titers
B) That their T cells lacked expression of CCR5
C) Dramatically increased numbers of NK cells compared to individuals with high HIV titers
D) That they were infected only with the X4 strain of the virus
E) Increased levels of chemokines that bind CCR5 compared to individuals with high HIV titers.

Question 22

In 1918, a worldwide epidemic of influenza A resulted in the deaths of 40-50 million people. This strain of influenza A, known as H1N1-referring to the genotypes of the viral surface proteins, hemagglutinin (H) and neuraminidase (N)-was shown to be derived from an avian virus that adapted to infect and grow in human cells. Interestingly, by 1957, the H1N1 strains of Influenza A had completely disappeared from the human population, to be replaced by new strains (H2N2) that contained three gene segments from avian origin. The most likely explanation for the disappearance of the early twentieth century form of H1N1 Influenza A virus is:

A) The replacement by a more pathogenic strain of Influenza A in 1957
B) The slow accumulation of mutations in the 1918 Influenza A virus that impaired its infectivity in humans
C) The changes in human living conditions leading to spread of Influenza A from pigs into humans, rather than from birds
D) High levels of existing immunity in the human population to the H1N1 surface antigens
E) The greater mobility of the human population due to the rapid increase in airplane travel

Question 23

Human genome-wide genetic association (GWAS) studies have identified several genetic polymorphisms that contribute to HIV control, and the rate of disease progression to AIDS. Which two immune cell subsets are implicated by these data as important regulators of HIV replication and the rate of disease progression?

Question 24

In the late 1990s, a group of individuals was discovered that remained uninfected with HIV, in spite of multiple exposures to the virus. Analysis indicated that these HIV-resistant individuals had a homozygous deficiency caused by a 32-bp deletion in a single gene, and furthermore, that this mutation was present at a frequency of ~10% (in heterozygous form) in individuals of European descent. These data provided clear evidence indicating that:

A) Healthy individuals can have a genetic deficiency in the expression of CD4.
B) Testing for HIV by seroconversion does not identify all infected individuals.
C) HIV infection requires a co-receptor in addition to CD4.
D) Some individuals can be infected with HIV but not develop AIDS.
E) Both R5 and X4 variants of HIV are equally important in HIV transmission.

Question 25

One HIV vaccine trial provided some small measure of hope for the development of an effective HIV vaccine. This vaccine, known as RV144, was administered to a high risk population in Thailand, and reduced new infections by ~30%. To address the mechanism of immune protection correlating with vaccine efficacy, a series of studies were performed. In one study, total IgG antibodies from four non-infected vaccinees were isolated and were mixed with HIV-infected cells plus peripheral blood lymphocytes from healthy unrelated donors. As a control, IgG antibodies were purified from four donors receiving the placebo, rather than the RV144 vaccine. Six hours later, lysis of the HIV-infected target cells was assessed, as shown in Figure Q40). Figure Q40) The immune mechanism that correlates with protection based on these data is:

A) CD8-dependent target cell lysis
B) Complement-dependent cell lysis
C) Induction of apoptosis by TNF-R stimulation
D) Target cell apoptosis induced by Fas-ligand stimulation
E) ADCC induced by NK cells

Question 26

Viruses utilize many different strategies to evade the immune response after infection. One strategy, illustrated in Figure , is an effect on virus-infected cells.
The virus used in this experiment is most likely:

A) An RNA virus with a segmented genome
B) A retrovirus
C) A negatively stranded RNA virus
D) A large DNA virus
E) A virus that establishes latency in the host

Question 27

HIV encodes several proteins that function to promote viral replication, packaging, and budding from host CD4 T cells. In addition to inhibiting cell-intrinsic antiviral mechanisms and down-regulating surface expression of MHC class I and class II molecules, these proteins function to:

A) Induce constitutive activation of NF- $\kappa$ B
B) Promote nuclear localization of NFAT
C) Block nuclear export of unspliced viral RNA
D) Up-regulate surface expression of CD4
E) Induce robust, sustained CD4 T cell activation

Question 28

The population of HIV viral variants circulating in the human population is extremely diverse, a phenomenon that contributes to the difficulties in generating an HIV vaccine. However, within a given individual, all HIV viruses are genetically identical.

Question 29

Herpesviruses are a class of viruses that establish life-long infections in human hosts. Estimates suggest that >90% of the population is infected with several of these viruses, including EBV, CMV, and herpes zoster, the cause of chicken pox. One herpesvirus, Herpes Simplex virus, causes recurrent cold sores in infected individuals. Thus, in spite of a robust anti-viral CD8 T cell response, these individuals still suffer from periodic virus-induced cold sores following exposure to UV light or certain hormones or other stressors. This anti-viral CD8 T cell response:

A) Eliminates infected epithelial cells but not infected neurons
B) Is unable to kill virus-infected cells
C) Produces IFN- $\gamma$ that has no effect on virus replication
D) Is prevented from trafficking to the site of virus replication due to virus-encoded chemokine receptor antagonists
E) Is inhibited by virus-encoded anti-inflammatory cytokines such as IL-10

Question 30

Studies have shown that secondary lymphoid tissues are a major reservoir of HIV in infected individuals. In part, this is due to the high numbers of viral target cells expressing CD4, such as T cells, macrophages, and dendritic cells. Surprisingly, secondary lymphoid tissues were also found to contain large numbers of infectious virus particles in the form of immune complexes. A comparison of the viral species found in these immune complexes indicates that they include virus particles that have been retained for over a year. The cells responsible for this reservoir of infectious HIV are:

A) HIV-specific B cells
B) Follicular dendritic cells
C) CD4 T cells
D) Subcapsular sinus macrophages
E) CD4⁺ dendritic cells

Question 31

Malaria is caused by protozoan parasites, Plasmodium falciparum and Plasmodium vivax. These pathogens are transmitted when an individual is bitten by a Plasmodium-infected mosquito. This route of transmission:

A) Allows the parasite to directly infect the red blood cells of the human host
B) Allows the infected blood cells of the mosquito to replicate and spread in the human host
C) Allows the parasite to replicate in the blood and rapidly spread all over the body
D) Prevents the parasite from being eliminated by the adaptive immune response
E) Allows the parasite to avoid the normal mechanisms blocking infections at barrier surfaces

Question 32

One important feature of retroviruses such as HIV is their generation of a provirus, a copy of the viral genome that is inserted into the host cell chromosome. In addition to providing a template for viral mRNA transcription, the proviral genome:

A) Is maintained long-term and transmitted to all of the cell's progeny
B) Is easily recombined out of the host cell chromosome to generate new viral genomes for packaging
C) Can be cleaved by the viral proteases to generate multiple viral proteins
D) Can induce DNA duplication to generate multiple viral copies in the host cell chromosome
E) Can be reverse transcribed to generate viral genomes for packaging into new virus particles

Question 33

HAART therapy is widely used in the US to treat HIV-infected individuals. This treatment is quite effective at inhibiting HIV replication, thereby preventing the progression to AIDS. However, HAART treatment is unable to completely eradicate all viral stores; consequently, patients must remain on this treatment indefinitely, as cessation of treatment leads to a rapid rebound in viral replication. One additional important side benefit of HAART treatment is its ability to:

A) Block HIV binding to CD4
B) Block HIV binding to CCR5
C) Greatly reduce HIV transmission
D) Stimulate CD4 T cell development in the thymus
E) Block viral reactivation from latency

Question 34

A common misconception is that our immune system fails to make a productive immune response to HIV infection, thereby leading to chronic infection. In fact, following a primary HIV infection, our immune system:

A) Generates a response, but it is an inappropriate Type II instead of a Type I response
B) Generates a response characterized by a cytokine storm that is poor at controlling virus replication
C) Generates a response that efficiently controls viral replication
D) Generates immunity at the mucosal surfaces, but does not control the systemic infection
E) Generates a T cell but not an antibody response to the virus

Question 35

The Borrelia hermsii spirochete causes a relapsing fever, characterized by multiple periods of high fever followed by asymptomatic intervals. The major outer membrane protein of the bacterium is known as VMP, for variable membrane protein, as >50 variants of this protein are found among
B. hermsii isolates. Interestingly, each bacterium contains genes encoding all of these variants, and during bacterial replication, ~1 in 10³-10⁴ bacteria undergo VMP switching per generation, thereby changing the VMP protein variant expressed on their surface. How does the VMP system explaining the relapsing/remitting fever caused by this pathogen?

Question 36

The first drug treatment for HIV licensed in the US was zidovudine (AZT), a reverse transcriptase inhibitor. However, AZT has now been completely replaced by HAART as the recommended treatment for HIV-infected individuals. The use of HAART, rather than AZT, is preferred because:

A) HAART includes a reverse transcriptase inhibitor that is more potent than AZT.
B) HAART only needs to be taken once per week compared to daily AZT treatment.
C) HAART has fewer side effects than AZT.
D) HAART is a combination therapy that reduces the possibility of viral escape mutants.
E) HAART is less expensive than AZT.

Question 37

HIV-infected patients that progress to AIDS suffer from a variety of opportunistic infections that rarely cause disease in healthy individuals. In addition, many of these patients acquire malignancies, such as B-cell lymphomas or Kaposi's sarcoma. A common feature of these two malignancies is that they are:

A) Each associated with a herpesvirus infection
B) Never seen in non-AIDS patients
C) The only two diseases caused by herpesviruses
D) Normally prevented by high affinity antibody responses
E) Normally eliminated by activated NK cells

Question 38

Listeria monocytogenes is a bacterial pathogen that causes a variety of diseases including gastroenteritis, encephalitis, and sepsis. The bacterium has evolved a strategy to replicate in the cytosol of macrophages, and to spread from one macrophage to another using the host's actin machinery to facilitate direct transfer between cells, thereby avoiding the extracellular space. This unique lifestyle of L. monocytogenes is dependent on the bacteria encoding enzymes that:

A) Lyse the phagocytic vesicle membrane, allowing bacterial escape into the cytoplasm
B) Prevent bacterial membrane proteins from activating pattern recognition receptors expressed in the macrophage
C) Block the MHC class II antigen presentation pathway
D) Block the MHC class I antigen presentation pathway
E) Induce the formation of granulomas that allow the bacteria to persist indefinitely in the host

Question 39

Evidence indicates that HIV-1 and HIV-2 originated in non-human primates, and only made the jump to infecting humans in the early twentieth century. As a consequence, these viruses are not well adapted to co-exist in equilibrium with their human hosts.

Question 40

In the case of HIV infection by sexual transmission, a key step in the establishment of disseminated HIV infection is the replication of the virus in regional lymph nodes draining the mucosal site of initial virus entry. The ability of the virus to spread from the mucosa to the regional lymph node is made possible by:

A) The high viral titers generated by the initial replication in the mucosal tissue
B) The virus' ability to infect migratory cells that carry it from the mucosa to the lymph node
C) The ability of infected CD4 T cells from the transmitting individual to proliferate and spread in the recipient's GALT
D) The extremely high titers of HIV required for transmission from one individual to the next
E) The resistance of virus in the mucosal tissue to elimination by antiretroviral drugs

Question 41

Listeria monocytogenes is a Gram-positive facultative intracellular bacterium. Infection with L. monocytogenes is the leading cause of food-borne illness. After uptake by phagocytic cells (e.g., macrophages) in the infected host, L. monocytogenes replicates to high levels in the macrophage cytosol, and then travels to neighboring macrophages by polymerizing cytosolic actin, and using these 'actin rockets' to traverse into an adjacent cell. A mutant strain of L. monocytogenes (Lm-mutant) has been identified that shows greatly reduced virulence in animal models, and is impaired in its ability to establish a focus of bacterial replication in isolated macrophages in culture, when compared to wild-type L. monocytogenes (wild-type Lm). Additional data indicated that wild-type Lm and Lm-mutant strains were equally efficiently phagocytosed by macrophages, indicating no defect in Lm-mutant bacteria in gaining access to their target host cell.
The graphs in Figure show: (top) % macrophages with large numbers of replicating bacteria after in vitro infection; (middle and bottom) CFU per liver or per spleen following infection of mice with non-lethal doses of bacteria. The table shows the LD₅₀, a measure of the dose of bacteria that will kill 50% of the mice following infection.
a) Based on these data, name two possible functions of the gene missing in the Lm-mutant strain.
To assess the function of the defective gene in the Lm-mutant strain, wild-type Lm and Lm-mutant bacteria were used to infect isolated macrophages and the subcellular localization of individual bacteria was examined by electron microscopy. Representative images of a timecourse post-infection are shown in Figure

b) What is the most likely function of the gene that is defective in the Lm-mutant strain and what is the fate of the Lm-mutant bacteria?
L. monocytogenes infection triggers a broad range of immune responses. Early after infection, L. monocytogenes induces innate immune responses consisting of the production of several pro-inflammatory cytokines and other mediators. This results in elimination of most of the bacteria. However, complete clearance of the L. monocytogenes infection requires an effective T cell response targeting the bacterial antigens. Thus, mice that lack T cells become chronic carriers of L. monocytogenes after infection.
c) Which subset of T cells is most likely to provide the protective immunity to wild-type L. monocytogenes?
d) Would you expect to see an altered T cell response to macrophages infected with Lm-mutant bacteria compared to those infected with wild-type Lm? Explain your reasoning

Question 42

Antiretroviral drugs are effective at blocking HIV replication in infected individuals, and in reducing the rate of HIV transmission. Physical barriers to transmission have also been shown to be effective at blocking HIV transmission. An additional strategy for reducing the rate of new HIV infections in high-risk populations utilizes a strategy to prevent the virus from establishing a permanent infection, once an individual has been exposed. This strategy utilizes:

A) Weekly injections of neutralizing anti-HIV antibodies
B) Pre-exposure prophylactic treatment with antiretroviral drugs
C) Condoms
D) Distribution of sterile hypodermic needles and syringes
E) Heat-killed HIV as a vaccine

Question 43

Common variable immunodeficiency (CVID) is a rather mild immunodeficiency disease that generally appears in young adults, with a peak onset between ages 20-30. Approximately 25% of the cases of CVID are familial, indicating a genetic basis for the disease. CVID patients suffer from recurrent infections of the respiratory tract, the ears, and the gastrointestinal tract. Common infections in these patients are caused by Haemophilus influenzae, Streptococcus pneumoniae and Staphylococcus aureus. Analysis of their peripheral blood cells shows normal numbers and subsets of T cells. B cell data from peripheral blood are shown in Figure :
a) In general, what is the immune mechanism most likely defective in patients with CVID? Explain your reasoning.
One particular family is chosen for further study. In this family, two of four children have CVID, as shown in Figure

Given this pedigree, the mother’s peripheral blood leukocytes are examined for their patterns of X-chromosome inactivation. The results show that all of her T cells, B cells, and monocytes show random (50:50) inactivation of each of her X chromosomes.
b) What do these data indicate about the gene defect causing CVID in this family?
In a second series of studies, peripheral blood cells from one CVID patient and one healthy child in this family were collected, and T cell and B cell populations were separately isolated. These cell populations were mixed in different combinations, and then cultured for one week in the cytokines IL-4, IL-21, and IL-6. At the end of one week, the concentrations of secreted antibody in the culture supernatants were measured

c) What do these data indicate about the gene defect causing CVID in this family?
In a final set of experiments, the isolated T cells and B cells from the study above were used in a transwell assay. In this assay, shown in Figure , cell populations are separated by a semi-permeable membrane. This allows soluble mediators to equilibrate between the top and bottom chambers, but prevents direct cell–cell contact.

As a control, the cell populations were mixed together in the bottom chamber, a result that would replicate the findings from the study above. In addition, in this study, all wells contained IL-4, IL-6, and IL-21. After 7 days, culture supernatants were analyzed for antibody concentrations.The results of the transwell experiments are shown in Table .

d) What do these data indicate about the function of the gene that is defective in the CVID patients from this family?