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HIV Pathogenesis: Immune Activation & Inflammation. HIV Research Catalyst Forum, April 21 2010. What is Immune Activation?. Normally a transient phenomenon, as with the flu or any other acute infection All the signs of activation happen, but only for a week or two

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hiv pathogenesis immune activation inflammation

HIV Pathogenesis: Immune Activation & Inflammation

HIV Research Catalyst Forum, April 21 2010

HIV Research Catalyst Forum April 21, 2010

what is immune activation
What is Immune Activation?

Normally a transient phenomenon, as with the flu or any other acute infection

All the signs of activation happen, but only for a week or two

Pathogen is then cleared or controlled, and activation subsides back to baseline

In HIV infection, activation subsides, but not back to baseline (even in most elite controllers)

Immune activation as measured by CD38 expression on CD8 T cells is the strongest predictor of the pace of disease progression

HIV Research Catalyst Forum April 21, 2010

t cell development
T-Cell Development
  • T cells are produced in the bone marrow then travel to an organ called the thymus that’s just behind your breastbone.
  • In the thymus, the T-cell acquires a “CD” surface marker that governs what type of T-cell it will be.
  • The two major T-cell types are:
    • CD4 helper T-cell.
    • CD8 T-cells, including cytotoxic T-lymphocytes (CTLs) or killer T-cells.

HIV Research Catalyst Forum April 21, 2010

t cell development1
T-Cell Development

Both CD4 and CD8 T-cells have a docking bay type structure called a "T-cell receptor" (TCR) that can dock with protein fragments called epitopes (from pathogens or other sources)

TCRs are generated in the thymus in a sort of fruit machine process that gives each T-cell one out of 25 million or so possible TCRs.

A newly made T-cell leaves the thymus to patrol around the body looking for an epitope that fits its TCR. At this stage the T-cell is called “naïve.”

HIV Research Catalyst Forum April 21, 2010

the immune response to infection
The Immune Response to Infection
  • On first exposure to a virus, incoming particles are taken up by the sentries of the immune system, dendritic cells (DC)
  • DCs can recognize pathogen-associated molecular patterns (PAMPs) shared by many different types of pathogens via toll-like receptors (TLRs)
  • DCs become activated (switched on) which causes them to migrate from the site of exposure to lymph nodes

HIV Research Catalyst Forum April 21, 2010

the immune response to infection1
The Immune Response to Infection
  • DCs break the pathogen down into protein fragments (called epitopes) which are then displayed on the outer surface by specialized molecules
  • Class II HLA (also known as MHC) molecules present epitopes to CD4 T cells
  • Class I HLA molecules present epitopes to CD8 T cells
  • In both cases recognition occurs via the docking bay structure on the outside of the cell, the T cell receptor (TCR)

HIV Research Catalyst Forum April 21, 2010

the immune response to infection2
The Immune Response to Infection
  • T cells travel through lymph nodes on string-like pathways made of fibroblastic reticular cells (FRC), these pathways form a complex traffic system with crossroads, junctions and dead ends
  • DCs hang out at crossroads like salesmen trying to interest T cells in the epitopes they have on offer

HIV Research Catalyst Forum April 21, 2010

getting activated
Getting Activated
  • A passing T cell that recognizes an epitope will engage in a prolonged embrace with the DC and eventually become activated

Video of DC (green) and T cell (red) interactions in a mouse lymph node, before and after injection of an antigen. Note how the red T cells only contact DC briefly until antigen is present, then prolonged contacts can be seen.

Celli et al. Immunity. 27:625-634

HIV Research Catalyst Forum April 21, 2010

getting activated1
Getting Activated
  • Activated T cells divide >15 times, generating a swarm of T cells specific for the same pathogen epitope
  • Dividing T cells switch on genes for making important signaling and antiviral proteins (chemokines & cytokines)
  • Inflammatory cyokines and rapid T cell expansion contribute to the symptoms during acute infection (fever, malaise, swollen lymph nodes)

HIV Research Catalyst Forum April 21, 2010

t cell subsets
T Cell Subsets
  • Different T cell subsets engage in different tasks, typically defined by production of particular cytokines
  • CD4 T cells
    • Type 1 (Th1): help CD8 T cells kill infected cells
    • Type 2 (Th2): support production of antibodies by B cells
    • Regulatory (Treg): release immune-suppressive cytokines to dampen the immune response
    • Th17: Recently discovered subset involved in responses to extracellular bacteria and autoimmunity

HIV Research Catalyst Forum April 21, 2010

t cell subsets1
T Cell Subsets
  • CD8 T cells
    • Recognize infected cells displaying pathogen fragments on their surface
    • Release cell-killing substances (perforin, granzyme B) that puncture the cell wall and destroy the infected cell

The CD8 T cell is the smaller cell at the bottom of the image that punctures a larger influenza virus-infected cell and destroys it.

HIV Research Catalyst Forum April 21, 2010

resolution memory
Resolution & Memory

When the infection is controlled, the majority of the newly-produced pathogen-specific “effector” T cells are no longer needed and die in a process called activation-induced cell death (AICD)

Importantly, a subset of pathogen-specific T cell and B cells survive and these are described as “memory” cells

Memory cells have enhanced functionality compared to naive cells and are often able prevent re-infection (with cleared pathogens) or control a pathogen that remains in the body (e.g. CMV, EBV, herpes zoster)

HIV Research Catalyst Forum April 21, 2010

slide14

Wherry & Ahmed, J. Virology, 78;11:5535-5545

HIV Research Catalyst Forum April 21, 2010

t cell pools
T-Cell Pools

Memory T-cell pool size

= ~200-300 billion

Naïve T-cell pool size

= ~100 billion

The thymus produces

around ~10-100 million

new naïve

T-cells every day

Naïve T cells that

meet a matching antigen

leave a legacy of

memory cells which

join the memory pool

Naïve T-cells that hadn’t

responded to anything

die to make room for

the fresh naïve T-cells

An existing memory

cell dies to make

room for the new

HIV Research Catalyst Forum April 21, 2010

acute hiv infection
Acute HIV infection

Transient (typically) loss of CD4 T cells from blood, significant loss of CD4 T cells from gut

High viral load

High levels of immune activation

Increased CD8 T cell counts & skewing of CD4:CD8 ratio

HIV Research Catalyst Forum April 21, 2010

acute hiv infection1
Acute HIV infection

HIV-specific immune responses become detectable in 2-3 weeks

Decline in viral load occurs in parallel with emergence of HIV-specific memory CD8 T cell response but is rarely fully controlled

Evidence of HIV-specific memory T cell dysfunction emerges early (HIV infection of developing CD4 responses may be the culprit)

Neutralizing antibodies are not generated for several months and are rarely able to neutralize contemporaneous virus

HIV Research Catalyst Forum April 21, 2010

hiv infects developing memory cd4 t cells
HIV Infects Developing Memory CD4 T Cells

CD127 aka IL-7R is a marker for T cells destined to become long-term memory cells

Zaunders et al

J. Virology, 80:20:10162-10172

HIV Research Catalyst Forum April 21, 2010

chronic hiv infection
Chronic HIV Infection
  • Immune activation persists
  • Immune responses to HIV become progressively more diverse
    • as the virus replicates, mutant forms arise and these induce new immune responses (from the naïve T cell and B cell pools)
    • effective immune responses pressure the virus to mutate in ways that prevent recognition, somewhat similar to the way HIV mutations can impair drug effectiveness (“immune escape”)

HIV Research Catalyst Forum April 21, 2010

chronic hiv infection1
Chronic HIV Infection
  • T cells become exhausted & senescent
    • lose the capacity to proliferate (copy themselves)
    • sequential loss of cytokine production capacity: IL-2>TNFalpha>interferon gamma
    • Express exhaustion markers (PD-1, Tim-3)
    • lose the CD28 co-stimulatory molecule, leading to an accumulation of CD28- T cells (also seen in aging)
    • Shortened telomeres
    • Dysfunctional HIV-specific CD4 and CD8 T cells accumulate

HIV Research Catalyst Forum April 21, 2010

parallels with aging
Parallels with Aging

Decreased thymic output

Decreased naive CD4 and CD8 T cell numbers

Decreased response to vaccinations

Skewed CD4:CD8 ratio

Narrowing of the T cell repertoire, particularly in CMV+ (memory pool gets crowded)

Increased numbers of CD28- CD8 T cells (associated with morbidity & mortality)

Increased levels of inflammatory cytokines (“inflammaging”)

HIV Research Catalyst Forum April 21, 2010

the immune activation suspects
The Immune Activation Suspects

Ongoing HIV replication: production of viral antigens and possibly also via viral HIV RNA stimulating toll-like receptors 7 & 8

Microbial translocation - leaking of normally harmless bacteria from the gut into the circulation, leading to increased levels of bacterial DNA and LPS in the bloodstream (also seen in idiopathic CD4 T cell lymphopenia, but it doesn’t cause CD8 T cell activation in that setting)

HIV Research Catalyst Forum April 21, 2010

the immune activation suspects1
The Immune Activation Suspects
  • Co-infections
    • hepatitis C co-infection associated with increased CD8 T cell activation
    • CMV: treatment with anti-CMV drug valganciclovir reduced CD8 T cell activation but did not increase CD4 T cell counts
    • Other herpesviruses (Epstein-Barr Virus, Herpes Simplex Virus types 1+2)
  • Loss of T cells leads to “homeostatic” proliferation

HIV Research Catalyst Forum April 21, 2010

impact of virus suppression
Impact of Virus Suppression

Immune activation declines rapidly

CD4 T cell increases: redistribution of cells trapped in lymph tissue, proliferation of functional cells, production of new naive T cells from the thymus (slowest aspect of recovery)

CD8 T cell numbers decrease

Memory T cell responses to opportunistic pathogens improve

HIV Research Catalyst Forum April 21, 2010

factors associated with poor cd4 recovery
Factors Associated with Poor CD4 Recovery

Low CD4 T cell nadir

Low naive/memory T cell ratio

Age

Immune activation

Microbial translocation

Hepatitis C co-infection (some conflicting data, largest studies find no significant effect)

Lymphoid tissue fibrosis

HIV Research Catalyst Forum April 21, 2010

long term cd4 recovery actg 384
Long Term CD4 Recovery – ACTG 384
  • Strata:
    • 1 (0–50)
    • 2 (51–200)
    • 3 (201–350)
    • 4 (351–500)
    • 5 (1500)

HIV Research Catalyst Forum April 21, 2010

long term cd4 recovery actg 3841
Long Term CD4 Recovery – ACTG 384

HIV Research Catalyst Forum April 21, 2010

poor cd4 recovery clinical risk
Poor CD4 Recovery & Clinical Risk

HIV Research Catalyst Forum April 21, 2010

cd4 boosting therapies
CD4 Boosting Therapies?
  • IL-2: Increased numbers but no clinical benefit (functionality?)
  • IL-7: Less toxic (so far), significant CD4 T cell increases, contribution of thymus to T cell increases still uncertain
  • Human Growth Hormone (HGH): Increased naive T cells & thymic output, significantly reduced CD4 and CD8 T cell activation, but not ideal (lower doses?)

HIV Research Catalyst Forum April 21, 2010

other immune based approaches
Other Immune-Based Approaches

CCR5 inhibitors e.g. maraviroc – multiple trials for poor CD4 recovery

Anti-PD1 antibodies – phase I for cancer

TLR7/9 inhibitors/agonists (chloroquine, Dynavax oligonucleotides) – ongoing ACTG trial of chloroquine vs. immune activation

HIV Research Catalyst Forum April 21, 2010

other immune based approaches1
Other Immune-Based Approaches

Rapamycin

Anti-CMV treatments – study in press

Flagyl-based antibiotics – one small recent study reported combination antibiotic treatment for H. Pylori improved CD4 recovery

Rheumatoid arthritis/inflammatory bowel disease treatments (IL-1 receptor antagonists, minocycline, leflonomide)

Anti-CTLA-4 – ongoing studies in cancer

HIV Research Catalyst Forum April 21, 2010

tx vaccines gene therapies
Tx Vaccines & Gene Therapies
  • Therapeutic vaccines
    • Better gut CD4 T cell recovery associated with HIV-specific CD4 T cell responses in the gut
    • Therapeutic immunization reported to decrease CD8 T cell activation
  • Genetically modified immunity
    • E.g. Carl June is extracting CD4 T cells, deleting the CCR5 gene, expanding & reinfusing in hopes of creating HIV-resistant HIV-specific T cells

HIV Research Catalyst Forum April 21, 2010

advocacy issues
Advocacy Issues
  • The generally dismal and scattershot IBT pipeline
  • Lack of trials or any other options for people with poor CD4 T cell recovery
  • Uncertainty about how to reduce activation/inflammation
  • Administrative obstacles to translational (bench-to-bedside) research
  • Lack of a systematic, coordinated research plan

HIV Research Catalyst Forum April 21, 2010