Evaluation of Human Thymic Function during Health and HIV1 Infection

1
Evaluation of Human Thymic Function during Health
and HIV-1 Infection

• Ping Ye and Denise Kirschner
• DIMACS Workshop
• 9/23/2001

2
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

3
The Thymus
Janeway CA et al. Immunobiology 5th ed. 2001
4
Thymopoiesis
Berkowitz RD et al. J Immunol. 161(7)3702-10,
1998
5
Recent Thymic Emigrants (RTE)

• 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

6
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
7
TREC as A Measure of RTE
8
Parameters Estimating RTE

• TREC concentration
• Is affected by both RTE and peripheral T cell
  proliferation and death
• Naïve T cells (CD45RA, CD62L, CD95)
• 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
• Assuming thymic volume is proportional to RTE
  levels

9
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?

10
Thymopoiesis Model
Cell Source
THYMUS TES(t)
TN
ITTP
DP
SP4
SP8
BLOOD/ LYMPHOID TISSUES
CD8 RTE
CD4 RTE
11
Simulation Results for Thymopoiesis Model
Thymocytes
RTE/day
12
Model Simulation Values Compared with
Experimental Data
13
TREC Model
THYMUS
CD4 RTE
CD8 RTE
CD4 T cell
CD8 T cell
TREC
TREC
BLOOD/ LYMPHOID TISSUES
14
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
15
Four Events Affecting TREC Concentration
THYMUS
SP4
SP8
1
CD4 RTE
CD8 RTE
2
3
CD4 T cell
CD8 T cell
4
TREC
TREC
BLOOD/ LYMPHOID TISSUES
1 Thymic output 2 T cell division 3 T cell
death 4 TREC degradation
TREC concentration is affected by
16
TREC Can Be Equally Affected by Thymic Output
and T Cell Division (at Any Age)

17
TREC Concentration during Aging
Healthy Volunteers
Age (years)
Douek DC et al. Nature. 396(6712) 690-5, 1998
18
Thymic Involution Induce TREC Decline (Over
Entire Lifespan)

• TREC concentration is a good marker for RTE in
  healthy people

19
SummaryThymic Function in Healthy people

• 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

20
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

21
HIV-1 Structure

• Associated with AIDS
• Retrovirus family
• Two identical ss RNA
• Enveloped virus
• Antigenic variation
• Infects CD4 cells
• Entry requires CD4 and coreceptor

22
HIV-1 Entry
23
Classification of HIV-1 Strains

• R5 strain
• Uses CCR5 as coreceptor
• Replicates slowly and is minimally cytopathic
• Is primarily transmitted
• Is prevalent in the early stage of disease
• X4 strain
• 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
• Uses either CCR5 or CXCR4 as coreceptor
• Has similar properties as the X4 strain

24
HIV-1 Disease Progression

• Hypotheses for the decline of CD4 T cells in the
  blood
• Changes in cell migration patterns
• Changes in cell life-spans
• Changes in cell production by the thymus

25
Clinical Studies of Thymic Infection with HIV-1

• 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

26
Thymic dysfunction (TD)

• Two distinct modes of pediatric HIV-1 disease
  progression
• Normal progressors 10 years to AIDS (85)
• Fast progressors 2-3 years to AIDS (15)
• lower CD4 and CD8 counts (DiGeorge
  Syndrome)

27
Experimental Studies of Thymic Infection with
HIV-1

• Coreceptors
• CCR5 Expressed in low levels on DP, SP4, SP8
  cells
• CXCR4 Expressed in high levels on ITTP, DP
  cells,
• low levels on SP4 and SP8 cells
• HIV-1 strains
• R5 strain Infects DP and SP4 cells
• Minimally cytopathic
• X4 strain Infects ITTP, DP and SP4 cells
• Highly cytopathic
• Thymocyte maturation stages
• SP4,SP8 Support active viral replication
• ITTP,DP Support less viral replication

28
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?

29
Thymic Model With HIV-1 Infection
Cell Source
TN
THYMUS TES(t)
X4
ITTPX
ITTP
DPX
DPR
DP
R5
X4
SP4R
SP4
SP4X
SP8R
SP8X
SP8
R5
X4
CD4 RTE
CD8 RTE
BLOOD/LYMPHOID TISSUES
30
Virtual Pediatric Thymic Infection
31
Simulations of CD4 RTE Predict Bimodal Pattern
of Blood CD4 T Cell Counts
32
Simulation Values at Year Two of Virtual
Infection Compared with Experimental Data
33
Bifurcation ParametersImplications for Treatment

• 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

34
TREC Concentration during HIV-1 Infection
HIV-1 Infected Subjects
Age
Age (years)
Douek DC et al. Nature. 396(6712) 690-5, 1998
35
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?

36
T Cell Kinetics during HIV-1 Infection
We include these effects in the model to
represent HIV-1 infection
37
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
38
Model Prediction for Thymic Output during HIV-1
Infection

• TREC concentration is a good marker for CD4 RTE
• TREC concentration is NOT a good marker for CD8
  RTE

39
SummaryThymic Function during Pediatric HIV-1
Infection

• 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

40
Conclusions

• 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

41
Publications

• 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).

42
Acknowledgments

• Dr. Denise Kirschner
• Collaborator Dr. Athena Kourtis
• Dr. Ramit Mehr
• Kirschner lab members
• Funding
• NIH and Whitaker Foundation to DEK
• Rackham Graduate School and Elizabeth Glaser
  Pediatric AIDS Foundation to PY

43
Clinical Studies of Thymic Function during HAART

• TREC levels recover
• Naïve CD4 and CD8 T cell numbers increase
• Thymic volume increases
• Children with thymic dysfunction response well
  to HAART

44
Model of Thymic Function Reconstitution during
HAART
Cell Source
TN
THYMUS TES(t)
X4
ITTPX
ITTP
DPR
DPX
DP
R5
X4
SP8R
SP4R
SP4
SP4X
SP8X
SP8
R5
X4
CD4 RTE
CD8 RTE
BLOOD/LYMPHOID TISSUES

• Reduce viral influx into the thymus
• Decrease viral infection and production (if
  thymic drug efficacy gt 0)
• Decrease HIV-1 induced death of uninfected
  thymocytes
• Increase progenitor cells from bone marrow
• Increase carrying capacity of the thymus

45
Simulations of pediatric thymocyte dynamics
during HAART
46
Simulations of CD4 RTE Predict HAART Agent
Thymic Efficacy
RTI
HAART