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?• 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


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.


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.”


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


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)


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


Getting Activated• A passing T cell that recognizes an epitope will engage in

a prolonged embrace with the DC and eventually become activated

Celli et al. Immunity. 27:625-634HIV Research Catalyst Forum April 21,

2010

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.

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)


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


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.

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)


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


T-Cell Pools


The thymus producesaround ~10-100 millionnew naïve T-cells every day

Naïve T-cell pool size= ~100 billion

Memory T-cell pool size= ~200-300 billion

Naïve T-cells that hadn’t responded to anything die to make room for the fresh naïve T-cells

Naïve T cells thatmeet a matching antigen leave a legacy of memory cells which join the memory pool

An existing memorycell dies to make room for the new

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


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 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


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”)


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


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”)


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)


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


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


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


Long Term CD4 Recovery – ACTG 384

• Strata:– 1 (0–50) – 2 (51–200) – 3 (201–350) – 4 (351–500) – 5 (1500)


Long Term CD4 Recovery – ACTG 384


Poor CD4 Recovery & Clinical Risk


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?)


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


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


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


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 pathogenesis: immune activation & inflammation

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