Evaluation of Human Thymic Function
during Health and HIV-1 Infection
Ping Ye and Denise Kirschner
DIMACS Workshop
9/23/2001
Outline
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Thymic function in healthy people
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Thymopoiesis
Markers to represent recent thymic emigrants (RTE)
Thymopoiesis model
TREC model
Thymic function during pediatric HIV-1 infection
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HIV-1 infection
Clinical and experimental studies
Thymic infection model
Evaluation of TREC
The Thymus
Janeway CA et al. Immunobiology 5th ed. 2001
Haynes BF. Clin Immunol. 92(1):3-5,1999
Thymopoiesis
Berkowitz RD et al. J Immunol. 161(7):3702-10, 1998
Recent Thymic Emigrants (RTE)
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Lack of phenotypic markers for human RTE
Human RTE generally refer to T cells that have
undergone only a few cellular divisions after
leaving the thymus
T cell receptor excision circles (TREC) have
recently been used as a measure of the number
of RTE
T Cell Receptor Excision Circles (TREC)
Pieces of DNA fragments that are generated during TCR gene
rearrangement in the thymus and then exported from the thymus to
periphery within T cells episomally
Janeway CA et al. Immunobiology 5th ed. 2001
Haynes BF et al. Annu Rev Immunol. 18: 529-60, 2000
TREC as A Measure of RTE
Total TREC level
Thymic output (RTE)
+
T cell division
NA
T cell death
TREC degradation
-
TREC concentration
+
+/-
Parameters Estimating RTE
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TREC concentration
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Naïve T cells (CD45RA+, CD62L, CD95+)
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Is affected by both RTE and peripheral T cell
proliferation and death
May have long lifespan
May proliferate in an antigen-independent manner
May rapidly convert to memory cells
May be converted from memory cells
Thymic volume
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Assuming thymic volume is proportional to RTE levels
Two Questions to Address
1st Can TREC concentration be properly used
as measure of RTE?
2nd Whether thymic infection with different
HIV-1 strains contribute to differences in
disease progression?
Thymopoiesis Model
Cell Source
THYMUS
TN
TES(t)
ITTP
DP
BLOOD/
LYMPHOID TISSUES
SP4
SP8
CD4+ RTE
CD8+ RTE
Simulation Results for Thymopoiesis Model
Thymocytes
RTE/day
Model Simulation Values
Compared with Experimental Data
SP4/SP8 versus CD4/CD8
Model value
SP4/SP8=2.0
Experimental data
SP4/SP8 = 2.0
CD4/CD8 = 1.5-2.5
Reference
Jamieson BD 99 (human)
Amadori A 96 (human)
RTEs/day (1 year)
RTEs/day / Total thymocytes
1.1x10 9
1.5%
~1x10 9
1.0-5.0%
Haynes BF 00 (human)
Shortman K 92 (mouse)
TREC Model
THYMUS
SP4
SP8
CD4+ RTE
CD4+
T cell
CD8+ RTE
CD8+
T cell
TREC
BLOOD/
LYMPHOID TISSUES
TREC
Simulation Results for TREC Model
Douek DC et al. Nature. 369(6712):690-5, 1998.
Zhang L et al. J. Exp. Med. 187(11): 1767-78, 1998
Four Events Affecting TREC Concentration
THYMUS
SP4
SP8
1
2
CD4+ RTE
CD4+
T cell
3
4
TREC
CD8+ RTE
CD8+
T cell
TREC
BLOOD/
LYMPHOID TISSUES
TREC concentration is affected by
1 Thymic output
2 T cell division
3 T cell death
4 TREC degradation
TREC Can Be Equally Affected by
Thymic Output and T Cell Division (at Any Age)
#
TREC Concentration during Aging
Healthy Volunteers
sjTREC
cjTREC
Age (years)
Douek DC et al. Nature. 396(6712): 690-5, 1998
Thymic Involution Induce TREC Decline
(Over Entire Lifespan)
TREC concentration is a good marker for RTE in healthy people
Summary
Thymic Function in Healthy people
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Our model quantifies the number of RTE and TREC
concentration over an 80-year lifespan
TREC concentration can be equally affected by thymic
output and T cell division at any age
Thymic involution induces TREC concentration decline
over the entire lifespan
TREC concentration is a good marker for both CD4+ and
CD8+ RTE in healthy people
Outline

Thymic function in healthy people





Thymopoiesis
Markers to represent recent thymic emigrants (RTE)
Thymopoiesis model
TREC model
Thymic function during pediatric HIV-1 infection




HIV-1 infection
Clinical and experimental studies
Thymic infection model
Evaluation of TREC
HIV-1 Structure
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Associated with AIDS
Retrovirus family
Two identical ss RNA
Enveloped virus
Antigenic variation
Infects CD4+ cells
Entry requires CD4
and coreceptor
HIV-1 Entry
Classification of HIV-1 Strains
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R5 strain
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X4 strain
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Uses CCR5 as coreceptor
Replicates slowly and is minimally cytopathic
Is primarily transmitted
Is prevalent in the early stage of disease
Uses CXCR4 as coreceptor
Replicates fast and is highly cytopathic
Is rarely transmitted
Appears in approximately 50% of patients in the late stage
of disease
R5X4 strain
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Uses either CCR5 or CXCR4 as coreceptor
Has similar properties as the X4 strain
HIV-1 Disease Progression
Hypotheses for the decline of CD4+ T cells in the blood
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Changes in cell migration patterns
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Changes in cell life-spans
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Changes in cell production by the thymus
Clinical Studies of
Thymic Infection with HIV-1
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TREC levels in CD4+ and CD8+ T cells decline
Naïve CD4+ and CD8+ T cell numbers decline
Thymic volume decreases
The thymus undergoes morphological changes
“Thymic dysfunction” (TD+) is described in a subset
of vertically-infected infants
“Thymic dysfunction” (TD+)
Two distinct modes of pediatric HIV-1 disease progression
•
•
Normal progressors
Fast progressors
10 years to AIDS (85%)
2-3 years to AIDS (15%)
lower CD4+ and CD8+ counts (DiGeorge Syndrome)
Experimental Studies of
Thymic Infection with HIV-1
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Coreceptors
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CCR5
CXCR4
Expressed in low levels on DP, SP4, SP8 cells
Expressed in high levels on ITTP, DP cells,
low levels on SP4 and SP8 cells
HIV-1 strains
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R5 strain
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X4 strain
Infects DP and SP4 cells
Minimally cytopathic
Infects ITTP, DP and SP4 cells
Highly cytopathic
Thymocyte maturation stages
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SP4,SP8
ITTP,DP
Support active viral replication
Support less viral replication
Two Questions to Address
1st Can TREC concentration be properly used
as measure of RTE?
2nd Whether thymic infection with different
HIV-1 strains contribute to differences in
disease progression?
Thymic Model With HIV-1 Infection
Cell Source
THYMUS
TN
TES(t)
X4
ITTP
R5
DPR
SP4R
SP8R
R5
X4
SP8
R5
R5
X4
DP
SP4
CD4+ RTE
BLOOD/LYMPHOID TISSUES
ITTPX
CD8+ RTE
DPX
SP4X
SP8X
X4
X4
Virtual Pediatric Thymic Infection
Simulations of CD4+ RTE Predict
Bimodal Pattern of Blood CD4+ T Cell Counts
Simulation Values at Year Two of Virtual Infection
Compared with Experimental Data
Measure
RTEINF X100%
RTENOINF
Infected thymocytesINF X100%
Total thymocytesINF
SP4
SP8
Pediatric Model
R5
X4
Data
63%
32%
31%-58%
10%-40%
0.009%
0.632%
0.001%-1%
0.005%
1.59
0.84
0.5
Reference
Zhang L, 99 (human TREC)
Douek DC, 99 (human TREC)
Baskin GB, 92 (monkey)
Muller JG, 93 (monkey)
Bonyhadi ML, 93 (SCID-hu mouse)
Bifurcation Parameters
Implications for Treatment
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Viral influx from blood into the thymus
Virulence of strains (viral infection rate and
production rate)
HIV-1 induced death of uninfected thymocytes
Progenitor cells from bone marrow
Carrying capacity of the thymus
TREC Concentration during HIV-1 Infection
HIV-1 Infected Subjects
sjTREC
Age
Age (years)
Douek DC et al. Nature. 396(6712): 690-5, 1998
Two Questions to Address
1st Can TREC concentration be properly used
as measure of RTE?
2nd Whether thymic infection with different
HIV-1 strains contribute to differences in
disease progression?
T Cell Kinetics during HIV-1 Infection
Event
Kinetics
T cell division CD4: ­ 1-2 fold
CD8: ­ 7-8 fold
T cell death
CD4: ­ 3-4 fold
CD8: ­ 3-4 fold
Reference
McCune JM 2000
Hellerstein M 1999
McCune JM 2000
Hellerstein M 1999
We include these effects in the model to represent HIV-1 infection
T Cell Counts and TREC Concentrations
in HIV-1 Infected Subjects (Age 30)
T cells
TREC
Margolick JB et al. J Acquir Immune Defic Syndr. 6(2): 153-61, 1993
Zhang L et al. J. Exp. Med. 187(11): 1767-78, 1998
Model Prediction for Thymic Output
during HIV-1 Infection
Event
Thymic output (RTE)
Kinetic prediction
CD4:  10-15 fold
CD8:  1-6 fold
Event
Kinetics
T cell division CD4: ­ 1-2 fold
CD8: ­ 7-8 fold
T cell death
CD4: ­ 3-4 fold
CD8: ­ 3-4 fold
Reference
McCune JM 2000
Hellerstein M 1999
McCune JM 2000
Hellerstein M 1999
TREC concentration is a good marker for CD4+ RTE
TREC concentration is NOT a good marker for CD8+ RTE
Summary
Thymic Function during Pediatric HIV-1 Infection
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Infection with R5 and X4 strains induces different degrees of
thymic dysfunction, which is related to CD4+ T cell counts and
disease progression as seen in pediatric patients
Thymic infection in pediatric patients is more severe than in adult
patients, likely due to higher viral loads and a more active thymus
Suppressing viral load in blood, high drug efficacy within the
thymus, and improving inherent thymic function are necessary for
reconstitution of RTE levels
Protease inhibitors have high levels of efficacy directly
suppressing viral replication within the thymus, while reverse
transcriptase inhibitors have low efficacy
TREC concentration is a reliable marker for CD4+ RTE, but not for
CD8+ RTE in HIV-1 infected subjects
Conclusions
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Peripheral T cell turnover should be examined together
with TREC concentration as a measure of RTE
Infection with different HIV-1 strains induces different
degrees of thymic dysfunction, contributing to CD4+ T
cell depletion and disease progression
With adequate suppression of viral replication within
both the blood and the thymus during HAART, thymic
function recovery can reconstitute immune cell numbers
Publications
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Ping Ye and Denise Kirschner (2002). Reevaluation of T cell
receptor excision circles as a measure of human recent thymic
emigrants. Journal of Immunology, 168(10), 4968-4979.
Ping Ye, Athena Kourtis, and Denise Kirschner (2002). The
effects of different HIV-1 strains on human thymic function.
AIDS Research and Human Retroviruses, 18(17) (In press).
Ping Ye, Athena Kourtis, and Denise Kirschner . Reconstitution
of thymic function in HIV-1 patients treated with highly active
anti-retroviral therapy (submitted).
Acknowledgments
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Dr. Denise Kirschner
Collaborator: Dr. Athena Kourtis
Dr. Ramit Mehr
Kirschner lab members
Funding
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NIH and Whitaker Foundation to DEK
Rackham Graduate School and Elizabeth Glaser
Pediatric AIDS Foundation to PY
Clinical Studies of
Thymic Function during HAART
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TREC levels recover
Naïve CD4+ and CD8+ T cell numbers increase
Thymic volume increases
Children with “thymic dysfunction” response well to
HAART
Model of Thymic Function
Reconstitution during HAART
Cell Source
TN
THYMUS
TES(t)
X4
ITTP
R5
DPR
SP8R
SP4R
R5
X4
SP8
R5
R5
X4
DP
SP4
CD4+ RTE
ITTPX
CD8+ RTE
DPX
SP4X
SP8X
X4
X4
BLOOD/LYMPHOID TISSUES
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Reduce viral influx into the thymus
Decrease viral infection and production (if thymic drug efficacy > 0%)
Decrease HIV-1 induced death of uninfected thymocytes
Increase progenitor cells from bone marrow
Increase carrying capacity of the thymus
Simulations of pediatric thymocyte
dynamics during HAART
Simulations of CD4+ RTE Predict
HAART Agent Thymic Efficacy
RTI
HAART