Symposium on T cell turnover and thymic function
By Ross Hewitt, MD for HIVandhepatitis.com
Why do CD4+ T cells decline over time in HIV infection? This very basic question generated a difference opinion among expert presenters at the symposium on T Cell Turnover and Thymic Function held at the 9th Annual Retrovirus Conference.
Viral and CD4+ T Cell turnover
David Ho led off the discussion by reviewing data collected over the past six years involving the dynamics of HIV replication and of both CD4+ and CD8+ T cells. Since the advent of potent antiretroviral (ARV) therapy, measurements have determined the half-life of an HIV virion to be only 30 minutes. Such techniques also have calculated the half-life of a productively infected CD4+ T cell to be approximately 1.5 days. So, if CD4+ T cells turnover so rapidly and vigorously, why does the CD4+ T cell count decline over time?
Models for T cell decline
The simplest model of what happens to T cells is that the total pool is contributed to with new T cells from several sources, such as the thymus and the bone marrow, and by proliferation of cells within the pool, and taken away by death or destruction of these cells. This simple model does not take into account that both CD4+ and CD8+ cells are at first naïve (not committed to a particular immune response target) and then become activated (committed to a target). So, the T cell pool must be split between both naïve and activated cells for CD4+ and CD8+ T cells alike. So, possible explanations for overall decline in T cells are: 1) fewer T cells are produced from the sources; 2) high rates of proliferation and death in HIV infection result in fewer T cells; and 3) T cells are destroyed at greater rates in HIV infection. Each presenter offered their unique take on these possible causes of T cell decline.
Perelson: CD4+ T cell decline is due to increased destruction
Alan Perelson, a collaborator of Dr. Ho, discussed results of experiments that used deuterated (“heavy”) glucose and measured its incorporation in the DNA of newly dividing, proliferating CD4+ and CD8+ T cells. His mathematical predictions assumed that the rate of new cells supplied by the sources and the rate of cell death or destruction was pretty much equal because CD4+ and CD8+ T cell counts do not change much over a short period of time. In HIV+ persons, the proliferation rates of CD4+ and CD8+ T cells were shown to be six-fold and eight-fold higher than in HIV negative persons. For CD4+ T cells, the source rate was four-fold higher while death rate was three-fold higher than in HIV-negative controls, but was unchanged for CD8+ T cells. He also noted that there is very rapid exchange between tissue compartments and the blood because labelled cells appear in the blood within 12 hours. After a year of HAART, a decrease of 50% in turnover occurs, practically normalizing turnover rates. He concluded that CD4+ T cell declines occur over time as a result of HIV destruction and not reduced sources since turnover is so high.
Kovacs: CD4+ T cell decline is due to increased destruction and decreased production
Joseph Kovacs presented data from another experimental technique to measure T cell turnover, incorporation of BrdU into the DNA of proliferating CD4+ and CD8+ T cells.
Like Dr.Perelson, he also found high turnover of both types of cells. He took his analysis one step further by studying whether the cells were naïve (or “resting”) or activated (“memory”). He found that the naïve cells were slowly proliferating and that the activated cells were rapidly proliferating.
Similar to Dr. Perelson, he found that HAART reduced the turnover of the activated cells but not the naïve cells. He also studied monocytes and found that while their rate of death or destruction was not affected by HIV infection or HAART, the source (bone marrow) was inhibited by HIV infection and improved with HAART.
He concluded that HIV destruction of CD4+ T cells could not account solely for the decline in these cells over time because the turnover is so high.
Miedema: CD4+ T cells decline due to increased destruction of naïve T cells
Frank Miedema offered a different perspective on the issue of the high T cell turnover that all of the presenters acknowledged did occur. His hypothesis is that the high T cell turnover had no direct relation to the issue of T cell decline over time. He noted that HIV infected cells express high levels of the activation marker Ki67. He studied a cohort of seroconverters over time and observed that people with lower CD4+ T cell counts prior to HIV infection have faster progression to AIDS, as do people with high immune activation.
Because naïve CD4+ T cells are activated as a result of HIV infection, he concluded that increased activation leads to increased consumption of naïve (resting) cells. Because naïve cells are the most difficult to regenerate, chronic activation will ultimately cause T cell depletion. This theory is further supported by the observed faster CD4 cell decline that occurs once a switch to syncytial inducing (SI) phenotype occurs. Such SI viruses, which predominately use CXCR4 as a co-receptor, are able to infect naïve resting cells as well as activated cells whereas non-syncytial inducing (NSI) viruses infect primarily activated cells.
He concluded that the natural pathway for the fate of a CD4+ or CD8+ T cell, thymus to naïve to activated (memory effector) to cell death, is accelerated because immune hyperactivation causes increased recruitment of naïve T cells along this pathway, resulting in T cell decline over time.
McClune: CD4+ T cells decline due to increased destruction and failure of compensatory mechanisms
Mike McClune offered the most unique perspective by reassessing what turnover experiments are actually measuring and then looked for other mechanisms to explain T cell decline.
Turnover experiments that assess the fractional changes in rates of turnover fail to take into consideration the size of the entire pool of cells. So while fractional rate of turnover is high, the total size of the cell pool is low and results in absolute turnover rates that are unchanged in HIV infection. The pool of naïve resting cells is maintained through division of progenitor cells.
In HIV infection, there is destruction of microenvironments for progenitor cell differentiation, namely in the bone marrow and the thymus. Surprisingly, the thymus is very abundant in some HIV+ adults (especially under age 39, with 350-500 CD4+ T cells) but thymic size is diminished in adults with late stage HIV disease.
Patients with an abundant thymus have enhanced T cell reconstitution upon treatment with HAART. This begs the question: Could the thymus be responsive to T cell loss and result in upregulation of T cell production? Two lines of evidence support this theory.
First, the cytokine IL-7, which is known to be important in thymic function, stimulates proliferation and inhibits apoptosis of lineage restricted progenitor cells. Levels of IL-7 rise when the CD4+ T cell count drops; IL-7 production is increased in lymph nodes in CD4+ T cell depleted patients; and the pre-HAART level of IL-7 predicts how high the CD4+ T count goes after HAART is administered.
Growth hormone also positively regulates T cell production. In five patients receiving growth hormone with HAART, all had an increase in thymic density and volume over six to 12 months. Growth hormone also increased the percentage of naïve circulating CD4+ T cells. He concluded that HIV causes accelerated destruction of T cells. Compensatory feedback pathways may help to sustain T cell counts but these pathways fail and T cells are not replaced. T cell decline over time occurs as a result of two sequential lesions: accelerated destruction of mature cells and then failure to regenerate.
So, why do CD4+ T cells decline over time in HIV infection? When looking again at the three possible explanations of reduced T cell production, high T cell turnover and increased T cell destruction, the data presented indicated that while turnover is high and probably the result of immune hyperactivation, turnover itself does not fully explain T cell decline.
HIV infection appears to result in hyperactivation, which results in increased rate of destruction of T cells. The lower T cell counts trigger feedback mechanisms which themselves become damaged by HIV infection and supply of new T cells dwindles resulting in overall decline of T cell counts over time. An important lesson from the session appeared to be that unless experiments directly measure the phenomenon in question, experiments that measure surrogate or indirect phenomenon depend upon assumptions about the relation to the phenomenon in question which may ultimately be incorrect and lead to erroneous interpretation.
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