On March 12th 2013 took place at Vall d’Hebron Institut de Recerca (VHIR) the seminar ‘Allogeneicity and Immunogenicity of Stem Cell Therapy: a cardiovascular focus’, conducted by Pr. Dominique Charron, MD, PhD, Professor of Medicine, Immunology at the University of Paris and Chairman of the Department of Immunology and Histocompatibility at Hospital Saint Louis in Paris. Research on stem cell therapies for regenerative medicine is progressing rapidly. Although the use of autologous stem cells is a tempting choice, there are several instances in which they are either defective or not available in due time. Allogenic stem cells derived from healthy donors presents a promising alternative. Whether autologous or allogenic, recent advances have proven that stem cells are not as immune privileged as they were thought. Therefore understanding the interactions of these cells with the recipient immune system is paramount to their clinical application. Transplantation of stem cells induces humoral as well as cellular immune response. The research group of Pr. Dominique Charron investigated the immune characteristics of human cardiac stem/progenitor cells lines in term of major histocompatibility complex (MHC) expression, allogenicity and immuno modulatory properties. By using an experimental model of allogeneic stimulation, they demonstrate that, whether under inflammatory conditions or not, human cardiac progenitor cells (hCPC) do not trigger conventional allogeneic T helper cells type responses but instead induce proliferation and selective expansion of suppressive of regulatory T cells (Treg). Thus, hCPC in allogeneic settings acquire the capacity to down-regulate an ongoing immune response. For stem cell therapy to move to the clinics, immunological barriers should be pragmatically managed. It could be recommended to minimize immunogenetic differences between stem cell and recipient, assay the immunization status of the recipient prior to stem cell injection, and monitor both allogenic and autoimmunity post stem cell transplantation. Integrating the unique immunobiology of stem cell with the patient immune status is key to successful translation to the clinics.

1. VHIR CONFERENCE 2013
BARCELONA
«Allogenicity & Immunogenicity of Stem
Cell Therapy : a Cardiovascular Focus »
DOMINIQUE CHARRON,MD,PhD
Dominique.Charron @ sls.aphp.fr
Laboratoire « Jean Dausset » & INSERM U 940
Hopital Saint-Louis ,IUH ,Université Paris-Diderot , France

2. IMMUNOGENETICS & MEDICINE
XXth Century
XXth Century
HLA,, MHC ,Cytokines,Receptors….
HLA MHC ,Cytokines,Receptors….
…TRANSPLANTATION, AUTOIMMUNITY,INFECTIONS
…TRANSPLANTATION, AUTOIMMUNITY,INFECTIONS
XXI st Century
HLA & MEDICINE (Schizophrenia,Parkinson … )
IMMUNO PHARMACOGENETICS
(Abacavir,Carbamazepin ,Allopurinol…)
REGENERATIVE MEDICINE/CELL & IMMUNO THERAPIES
TOWARD
« SYSTEMS BIOLOGY/SYSTEMS MEDICINE »

3. MHC / HLA
Complete sequence
and gene map
HLA CONSORTIUM – Nature 11/1999

4. # HLA Alleles

5. HLA DIVERSITY = BIOLOGICAL SELF
HLA DIVERSITY = BIOLOGICAL SELF
2013 : 8496 ALLELES
WE ARE THE LIMIT

6. Pathways of Allorecognition
Pathways of Allorecognition
Tc Th
Cytotoxic T-cell Helper T-cell
Allogeneic (Donor) Cell
Allo MHC molecules MHC or other molecules
from the Donor are are shed
recognized by Host T- taken up and
Tc
Class I Class II processed by host APC
cells Cytotoxic T-cell
Allogeneic (Donor) APC
(stimulator)
Direct allorecognition
Direct allorecognition Th
Helper T-cell
Host APC
(recipient)
Peptide derived from allo molecules
presented on host MHC
Indirect allorecognition
Indirect allorecognition to Host T-cells
Immune Cell
Stem Cell

7. Allo Recognition/Activation Pathways
Direct vs Indirect
Recognition phase
APC Donor Recipient
HLA-peptide Donor -Donor Recipient - Donor
Pre-existing
T cell frequency 1/103 – 1/104 1/105 – 1/106
Immediate Response
Effector phase
Cytokine production
(inflammatory response) + ++
Th B cell activation
(Ab production) - +++
Direct cytotoxicity
Tc (of donor tissue) ++ -
Duration of response
Short lived long lived
(donor APC) (donor peptide)
Outcome Acute Rejection Chronic Rejection

8. CONSEQUENCES OF HLA HISTO- INCOMPATIBILITY IN
TRANSPLANTATION ……2012
T CELL MEDIATED : REJECTION (ORGANS)
& GVH(HSCT)
ANTI HLA ANTIBODY MEDIATED: CHRONIC
REJECTION(ORGANS) & NO ENGRAFTMENT (HSCT)
2013 CHANGE OF PARADIGM

10. Antibody-mediated vascular rejection of kidney allografts:
a population-based study
Carmen Lefaucheur*, Alexandre Loupy*, Dewi Vernerey, Jean-Paul Duong-Van-Huyen, Caroline
Suberbielle, Dany Anglicheau, Jérôme Vérine, Thibaut Beuscart
, Dominique Nochy, Patrick Bruneval, Dominique Charron, Michel Delahousse, Jean-Philippe
Empana, Gary S Hill, Denis Glotz, Christophe Legendre, Xavier Jouven
LANCET Nov 23,2012

11. Population based study
2079 patients(nck/sls)+ 602validation samples(foch)
302 biopsy proven rejection
(1998-2008)
CINICAL, HISTO PATHOLOGICAL(including C4d)&
IMMUNOLOGICAL(DSA) DATA
Hierarchical cluster analysis
unsupervised principal component
4 patterns of rejection
TCMR/V+ :T cell mediated rejection (26 9 °/°)
ABMR/V+ :Antibody mediated rejection(64 21°/°)
TCMR/V- : T cell mediated rejection without
vasculitis(139 46°/°)
ABMR/V- : Antibody mediated rejection without
vasculitis(73 24°/°)

12. PATHOLOGICAL & IMMUNOLOGICAL PHENOTYPES OF THE 4
REJECTION PATTERNS

13. Cellular (Tcell) rejection Antibody mediated rejection
TCMR V - ABMR V
-
Endarteritis --
Endarteritis +
ABMR V +
TCMRV+

14. GRAFT SURVIVAL IN THE 4 REJECTION
PHENOTYPES
ABMR V+

15. HLA, MHC AND MUCH MORE….
HLA, MHC AND MUCH MORE….
…TRANSPLANTATION, AUTOIMMUNITY AND MUCH MORE
…TRANSPLANTATION, AUTOIMMUNITY AND MUCH MORE
HLA in MEDICINE
IMMUNOPHARMACOGENETICS
REGENERATIVE MEDICINE
SYSTEMS BIOLOGY

16. REGENERATIVE MEDICINE AND TRANSPLANTATION
BENEFITS
BENEFITS
 PLURI //MULTIPOTENCY
 PLURI MULTIPOTENCY
 SELF RENEWAL
 SELF RENEWAL
 IN VITRO SPECIFIC DIFFERENCIATION
 IN VITRO SPECIFIC DIFFERENCIATION
 IMMUNE PRIVILEGE ?
 IMMUNE PRIVILEGE ?
LIMITS OF IN VIVO ENGRAFTMENT AND FUNCTIONALITY
LIMITS OF IN VIVO ENGRAFTMENT AND FUNCTIONALITY


IMMUNOGENICITY/ALLOGENICITY/REJECTION/AUTOIMMUNITY ?
IMMUNOGENICITY/ALLOGENICITY/REJECTION/AUTOIMMUNITY ?
 DISPONIBILITY – TIMELINE
 DISPONIBILITY – TIMELINE
 AGING
 AGING
 SAFETY
 SAFETY
 ETHICAL – REGULATORY ISSUES
 ETHICAL – REGULATORY ISSUES

17. THE IMMUNITY FACTORS IN REGENERATIVE
CELL THERAPIES
 THE IMMUNOGENETIC FACTOR: ALLOGENICITY
HLA, MHC and Much More….
 THE IMMUNE EFFECTORS: DIRECT vs INDIRECT PATHWAYS
OF ALLO RECOGNITION
Cells, Mediators and Allo Antibodies...
 THE AGING FACTOR: IMMUNO SENESCENCE
Towards an IMMUNOLOGICALLY EDUCATED CHOICE OF
SCs
Immune Cell
Stem Cell

18. 2002 -2008
ALLOGENEIC STEM CELLS ARE NOT
ALLOGENEIC STEM CELLS ARE NOT
IMMUNO PRIVILEGED
IMMUNO PRIVILEGED
MHC EXPRESSION
IMMUNOGENICITY INCREASES
UPON DIFFERENCIATION
IN VIVO REJECTION
Immune Cell
3 SUPPORTING PAPERS
Stem Cell

19. CHARACTERIZATION OF THE EXPRESSION OF MHC PROTEINS IN
HUMAN EMBRYONIC STEM CELLS
M. DRUKKER, G. KATZ, A. URBACH, M. SCHULDINER, G. MARKEL, J. ITSKOVITZ-ELDOR,
B. REUBINOFF, O. MANDELBOIM, N. BENVENISTY
PNAS, 2002, 99:9864
UNDIFFERENTIATED DIFFERENTIATED
In vitro In vivo
β2m
Counts
HLA-I
721/HLA-G
HLA-II
Immune Cell
Stem Cell
Fluorescence intensity

20. IFN-γ induction of MHC-I in human ES cells is dose and
IFN-γ induction of MHC-I in human ES cells is dose and
time dependent
time dependent

21. Embryonic Stem Cell Immunogenicity Increases Upon Differentiation
After Transplantation Into Ischemic Myocardium
R-J Swijnenburg, M. Tanaka, H. Vogel, J. Baker,T. Kofidis, F. Gunawan, D.R. Lebl, A.D.
Caffarelli, J.L. de Bruin, E.V. Fedoseyeva, R.C. Robbins
Circulation. 2005;112:I-166-I-172
Graft infiltration of immune cells after transplantation of in vivo differentiated ESCs
T cells B cells

22. 2O10
DIFFERENCIATION OF ALLOGENEIC MESENCHYMAL STEM CELLS INDUCES
IMMUNOGENICITY & LIMITS THEIR LONG-TERM BENEFITS FOR MYOCARDIAL REPAIR
Xi-Ping Huang & coll Circulation .2010 ;122:2419-242
• Wistar and lewis rats
• MSCs untreated vs MSCs cultured with 5-azacytidine(to induce myogenic
differentiation)
Flow cytometric & mRNA evaluation of MHC Ia,II and CD86 is increased by
>30% upon differentiation While MHC Ib is decreased
-------GFP+ MSCs Implanted into the infarcted myocardium 3 weeks after MI
express low level of MHC Ia when undifferenciated(alpha-SMA-) at day
seven & high level of MHC Ia when differenciated(alpha-SMA+) at Day
14(differenciated)
------ Implanted Allogeneic MSCs induce a local immune reaction after 7 days
and are not detected in situ after 5 weeks
-------Allogeneic MSCs restore cardiac function as effectively as Syngeneic
MSCs for 3 months but not 6 monts after implantation
While immunoprivileged in their undifferenciated state MSCs
become immunogenic in vitro & in vivo when differenciated
(biphasic immune response)

23. ALLOGENICITY/IMMUNOGENI
CITY &
IMMUNOMODULATORY
PROPERTIES of HUMAN
CARDIAC
PROGENITOR/STEM CELLS

24. Human Cardiac Stem/Progenitor Cells Characterization
•Cells from Different donors : Endomyocardic biopsy
Collagenase Treatment ckit purification/enrichment • Pluripotency Transcription factors
STEM CELL MARKERS
hCPC Cardiac differenciation
hCPC Cardiac differenciation
potency(in vitro)
potency(in vitro)
Cardiomyocytes
Cardiomyocytes
• CARDIAC LINEAGE SC MARKERS Endothelial cells
Endothelial cells
Smooth muscle cells
Smooth muscle cells
Nadal-Ginard B et al. Resident
human cardiac stem cells: role in
cardiac cellular homeostasis and
potential for myocardial
regeneration. Nat Clin Pract
Cardiovasc Med 2006;3

25. Immunology of Human Cardiac Stem/Progenitor Cells
for cardiac repair
3% O2
Non- Inflammatory
Inflammatory (IFNγ 72h)
 IL-10
producing
CD4+ cells
 low immunogenic profile
Lauden L. et al, Circ Res, 2013

26. Immunology of Human Cardiac Stem/Progenitor Cells
for cardiac repair
hCPC are Immuno-modulators
 Down-regulate an ongoing immune response (CD4+, CD8+, IFNγ, IL-2)
 More potent within inflammatory conditions
Lauden L. et al, Circ Res, 2013

27. hCPC & Regulatory T cells
ALLOGENEIC hCPC ACTIVATE AND EXPAND T regs
ALLOGENEIC hCPC ACTIVATE AND EXPAND T regs

28. Characterisation of hCPC activated Regulatory T cells
HLA DR+ PD-1+ Regulatory TTCells
HLA DR+ PD-1+ Regulatory Cells

29. PD-L1 (1)
anti-PD-L1
anti-PD-L1
Inhibition of Treg expansion
Inhibition of the
Immunomodulatory
effect

30. PD-L1 (2)
siRNA Inhibition
siRNA Inhibition
PD-L1 is implicated in Treg ss activation & immunomodulation by hCPCs
PD-L1 is implicated in Treg activation & immunomodulation by hCPCs

31. Conclusions
••hCPC are attractive for clinical Translation
hCPC are attractive for clinical Translation
-- Low immunogenic profile( No immediate rejection ? ))
Low immunogenic profile( No immediate rejection ?
--Maintainanceof immunologic properties under inflammatory
Maintainance of immunologic properties under inflammatory
conditions
conditions
--Immunomodulatoryproperties
Immunomodulatory properties
••PD-L1/PD-1 orchestrate immunologic properties of hCPC
PD-L1/PD-1 orchestrate immunologic properties of hCPC
--Treg generation
Treg generation
--hCPC-induced immunomodulation
hCPC-induced immunomodulation
••Allogenic-driven benefit?
Allogenic-driven benefit?
--hCPC-induced allogenic response is biased towards Treg
hCPC-induced allogenic response is biased towards Treg
--Immune properties of hCPC are controlled by PD-L1/PD-1 pathway
Immune properties of hCPC are controlled by PD-L1/PD-1 pathway

32. ALLOGENICITY
Beneficial Detrimental
Allogenic-driven-risk versus Allogenic-driven-benefit
 Immune behavior of other stem cells (iPSC and ESC- and iPSC-derived progenitors)
 Regulation of MHC expression in stem cells and their progenitors
 Reactivity with allo-antibodies (risk or benefit?)
 Markers of selection (PD-L1?)
Immunologically educated choice of
ALLOGENIC STEM CELLS
Optimization of ALLOGENIC STEM CELLS for use
in Regenerative Medicine

33. 2002 - 2010
 Allogeneic ESCs are Immunogenic : alloimmunity
2010 - 2012
Reprogrammed iPSCs are immunogenic :autoimmunity
Gene Transduced cells are immunogenic: autoimmunity
Allogeneic MSCs are immunogenic:alloimmunity
Endomyocardiac stem cells are allogeneic and
Immunomodulatory
Immune Cell
Stem Cell

34. • IMMUNOGENETIC SELECTION/BANK OF
SC/MATCHING
• INDUCTION OF TOLERANCE
• IMMUNOMODULATION
• IMMUNOMONITORING

35. Immunogenetic selection
– HLA MATCHING - REDUCING HLA MISMATCHING
• Pre TX Screening for anti HLA
Characterizing anti HLA specificities(SAB)
+ MIC-A? C1Q? C4d?
Cross-Matching
Post TX monitoring anti HLA antibodies
STEM CELL BANKING
Autologous Cells (anticipatory)
Cells derived from Homozygous individuals
(frequent haplotypes)
Allogenic Cells

36. A transplant immunologist point of
view
• Minimize immunogenetic differences
• Assay the immunization status of the
recipient prior to SCT injection
• Monitor allogenic & autoimmunity post
SCT
• Be pragmatic (…immunosupression) and
• Utopic( …hope for tolerance one day )

37. Acknowledgement
Reem AL DACCAK
Hopital Saint-Louis
Khaoussou Sylla UMRS940
Isabelle Martins
Kiran Ramgolam
Laura Lauden IC
Wahid Boukouaci
TC&SC
Luis Borlado
Miguel Mulet Parada
Ryad Tamouza Coretherapix (Madrid, Spain)
Caroline Suberbielle Bernardo Nadal-Ginard
Georgina Ellison
Pascale Loiseau Liverpool John Moores University
Emeline Masson (Liverpool, UK)
FP7 – CARE-MI Consortium
, INSERM; EU FP7 – CARE-MI

38. HLA IMMUNITY & STEM CELL THERAPY 2013
NOW THIS IS NOT THE END
IT IS NOT EVEN THE BEGINNING OF THE END
BUT IT IS PERHAPS,
THE END OF THE BEGINNING…
Hopital Saint-Louis

40. Immune phenotype: HLA expression
Immune phenotype: HLA expression
HLA-class I HLA-DR HLA-DQ HLA-DP
hCSC (850) (7) (0) (18)
CD80 CD86 CD40
(0) (33) (0)
hCSC-IFNγ
HLA-class HLA-DR HLA- HLA-
I (405) DQ DP
(18400) (540) (540)
MHC II… induction
only seen when cells are maintained under
hypoxia (3% O2)
physiological inflammatory conditions…?

41. hCPC IMMUNOPHENOTYPE
RESTING & UNDER INFLAMMATORY CONDITION (INF g treated)
No change in morphology
& pulripotency markers
after IFNγ
treatment
hCPC display a low immunogenic profile
Induction of HLA-class II
+ INF G Increase expression of HLA-class I & PD-L1

42. Immunology of Human Cardiac Stem/Progenitor Cells
for cardiac repair
Allogenic Human Cardiac Progenitor Cells
 Low immune risk even within inflammatory environment
 Reparatory by promoting Treg and by their expression of PD-L1
*Regulatory T cells are implicated in cardiac repair after Myocardial infarction
Tang TT, et al. Cell Physiol Biochem. 2010; Dobaczewski M, et al. Am J Pathol. 2010; Tang TT, et al. Basic Res Cardiol. 2012
*Myocardial PD-L1/PD1 control immune-mediated cardiac injury and polymorphonuclear inflammation
Grabie N, et al. Circulation. 2007;116:2062
PD-L1-Dependent Allogenic-Driven Benefit
Promotes
Clinical Translation Highlights PD-L1
marker
 NK cells response
Identify & Select
 Reactivity with anti-HLA antibodies
Low-Risk/High-Benefit
allogenic cardiac repair cells

43. hCSCs-triggered Allogenic Response
CD4+ T cells

44. hCSCs immune-modulation of an ongoing T cell
response
PHA CSCs +
alone Allo PBMC CSCs IFNγ MSCs
T cell proliferation
89.1% 95.3% 71.6% 70.4% 28.1%
CD4
90.7% 97.1% 69.6% 71.6% 19.6%
CD8
CFSE

45. CSCs can be recognized by anti HLA allo-antibodies
CSCs
PBMC

46. SUMMARY
SUMMARY
 CSCs express MHC I and MHC II under physiological &
inflammatory situation
 They display higher ALLOGENICITY than MSCs and are
less powerful in down regulating an ongoing immune
response
 CSC capacity to induce or modulate an immune response
is variable depending on both donor and recipient
 CSCs are recognized by allo-anti-HLA sera, which can
lead to in their elimination but could also be part of their
paracrine effect
Circulation Research 2012(in press)

48. Immunosuppressive Therapy Mitigates Immunological Rejection of Human
Embryonic Stem Cell Xenografts
R.J SWIJNENBURG, S. SCHREPFER, J.A. GOVAERT, F. CAO, K. RANSOHOFF, A.Y SHEIKH, M. HADDAD,
A.J CONNOLLY, M.M DAVIS, R.C ROBBINS, J.C WU
PNAS, 2008,105:12991
IN VIVO VISUALIZATION OF
HESC SURVIVAL
Immune Cell
Stem Cell

49. hCPC induced T Cell Response
Allogeneic co culture: CFSE Labelled PBMC/ Mitomycin-C treated hCPC
Allogeneic co culture: CFSE Labelled PBMC/ Mitomycin-C treated hCPC
Low IFNγ & IL-2 production
Low response similar to MSC induced High IL10 Production
hCPC induce low allogeneic T-cell response
hCPC induce low allogeneic T-cell response

50. 2012 Human cardiac Stem Cells characterization
Cells from three different CH1, CH3, CH4
donors Endomyocardic Biopsy
c-kit
(48)
CD90
(2880)
SSEA-1
(269) Collagenase treatement
C-kit purification/enrichment
γ
Cell counts
IFN
Cs
SC
M
C-
C
PB
CS
M
CS
SSEA-4 CD166
CD44
(1779) (100)
(14955) Oct4
Sox2
Nanog
Fluorescence intensity β-actin
DAPI DAPI DAPI
NKx2.5 oct4 SSEA-
4
Cardiomyocyte
DAPI DAPI DAPI
FITC- PE-IgG FITC-IgG Cardiac differenciation Endothelial
IgG potency
Smooth muscle
(In vitro)
Nadal-Ginard B et al. Resident human cardiac stem cells: role in cardiac cellular homeostasis
and potential for myocardial regeneration. Nat Clin Pract Cardiovasc Med 2006;3

51. Immunology of Human Cardiac Stem/Progenitor Cells
for cardiac repair
HLA-DR+ PD1+
Regulatory T cells
hCPC in allogenic settings have the ability to induce tolerance, by
promoting allo-stimulation and a contact and PD-L1-dependent
regulatory response
Lauden L. et al, Circ Res, 2013

52. IN THE NEWS 2011
• INDUCED PLURIPOTENT STEM CELLS(iPS)
ARE IMMUNOGENIC
&
RESULT IN AUTO-IMMUNITY

53. Immunogenicity of induced pluripotent stem cells
T. Zhao, Z-N Zhang, Z. Rong and Y. Xu
Nature, 2011, doi:10.1038/nature10135
* Proof of Principle
Teratomal Immune
Rejection/Tumor Regression
ESCs from C57BL/6 B6 (IR/TR)
0
ESCs from 129/SvJ B6 +++
* Reprogramming of of B6 Embryonic Fibroblast (MEF)
Oct4,Sox2,Klf4/Oct4,Sox2,Myc, Klf4
Retroviral Approach Episomal Approach
ViPSCs EiPSCs
IR/TR +++ B6 mice ++
Immune Cell CD4 T cell infiltration/Cell necrosis
Stem Cell

54. Mechanisms??
* Gene profiling of EiPSCs
Major findings
* episomal vector is deleted
* abnormal overexpression of endogenous genes
- Hormad 1 (tumor specific Ag)
- Spt1 (tumor specific Ag)
- Zg16
* Immune Rejection/Tumor Rejection
Major findings
* CD4 CD8 mediated immune rejection through Ag
specific (Zg16, Hormad1) T cells
- confirmed by in vivo purified T cells (Ag specific)
detected by (IFNγ release
assay) upon co-culture with
Ag-transfected DC from B6 mice
Conclusion
Break of Peripheral Tolerance/Expression of Minor Antigens
Immune Cell
Stem Cell

55. Clinical translation can only be successful if good knowledge of
biological processes linked to the therapeutic effect exists
Cancer Stem Cells Stem Cells
Pluripotency
Self-renewal
Plasticity
Immune-modulation
Factors promoting their Immune Behavior
Common Distinct
(PD-L1?, MHC II?)
How these cells are Tolerated to persist
Biomarkers
Immune protocols
Targets
Eliminate Engraftment
Combat Repair

56. hCPC Immunomodulatory property
PHA polyclonal stimulation
PHA polyclonal stimulation
hCPC are capable to modulate an ongoing Immune response
hCPC are capable to modulate an ongoing Immune response

57. the function of MHC II molecules is not limited to their role as antigen-presenting
structures; they are receptors that by triggering a variety of signaling pathways can
regulate cells activities from proliferation and maturation to apoptosis
Could a signal via MHC II or I contribute to the Regenerative
PARACRINE effect of allogenic CSCs???

58. SUMMARY
SUMMARY
 CSCs express MHC I and MHC II under physiological &
inflammatory situation
 They display higher ALLOGENICITY than MSCs and are
less powerful in down regulating an ongoing immune
response
 CSC capacity to induce or modulate an immune response
is variable depending on both donor and recipient
 CSCs are recognized by allo-anti-HLA sera, which can
lead to in their elimination but could also be part of their
paracrine effect
Circulation Research 2012(in press)

59. Additional Challenges
• Injured myocardium is
inflammatory:Immune reactivity increase
• Presence of APC within the differenciated
cell population(Endothelial ,Dentritic cells
…)
• MHC class I expression :NK cells
susceptibility vs Cytotoxic T lymphocytes