Transplantation and immunology

Question 1

Definitions of transplant relationships

Answer 1

• Autologous: self
• Syngenic: identical twin or clone
• Allogenic: non-identical (matched related or unrelated, cord blood, haploidentical)

Question 2

3 types of transplant rejection

Answer 2

• Hyperacute (first couple days): preformed Abs bind to graft and cause complement activation, leading to damage and thrombosis of vessels within the graft
• Acute (cellular or Ab mediated)
• Ab mediated: 3 types, all involve circulating Abs that activate complement (Cd4), can be arterial or capillary involvement (can be w/ or w/o inflammation)
• T cell mediated: 5 types, all result in inflammation of the vessels w/in the transplanted organ, this leads to fibrosis of the vessels
• Chronic: the 5th type of T cell mediated rejection, with continual inflammation and fibrosis

Question 3

How to prevent rejection

Answer 3

• Closely match donor and recipient
• Removed preformed Abs from recipient to prevent hyper acute rejection
• Block co-stimulatory molecules (CD28)
• Immunosuppressive medications

Question 4

Complications of immunosuppressive drugs

Answer 4

• Infections, malignancies
• Viral infections: CMV, EBV, PTLD (post transplant lymphoproliferative disease), BK virus
• BK virus: immunosuppression leads to reactivation (80% have the virus), BKV nephropathy is responsible for at least 50% of graft loss
• Other infections: fungal, bacterial
• Malignancies: EBV lymphoproliferative disease, kaposi’s sarcoma, squamous cell CA

Question 5

Cell types involved in alloreactivity

Answer 5

• Passenger leukocytes: MHCII bearing donor Ags w/in the graft
• Alloreactive helper T lymphocytes: recipient CD4 lymphocytes that recruit and activate macs
• Alloreactive CTLs: recipient CD8 Ts that lyse graft cells
• B lymphocytes: donor derived B lymphocytes that produce alloAbs and bind to graft endothelium

Question 6

Immunosuppression (IS) regimens

Answer 6

• Induction: basiliximab and anti-thymocyte globulin
• Maintenance: steroids, tacrolimus or cyclosporin, MMF or azathioprine, sirolumus, rituximab
• Side effects of IS: non immune toxicity to other tissue, consequence of IS for infection and malignancy

Question 7

Basiliximab

Answer 7

• Anti-CD25 monoclonal Ab
• Used in patients w/ low-moderate risk of rejection
• Expression of CD25 requires T cell activation
• CD25 is IL2 binding site, blocking this prevents further activation and proliferation of T cells
• Causes little depletion and has minimal toxic effects

Question 8

Antithymocyte globulin (ATG)

Answer 8

• Monoclonal IgG from other species, directed against T cells
• Provides a profound and durable lymphopenia
• May cause thrombocytopenia, cytokine release syndrome, serum sickness (type III hypersensitivity), allergic rxn

Question 9

Cyclosporin

Answer 9

• Binds to cyclophilin, which binds to calcimodulin and calcineurin to form a complex of all 4
• This prevents initial activation of T cell via TCR signaling pathway
• Side effects (dose-dependent): nephrotoxicity, HTN, hyperlipidemia, gingival hyperplasia, hirsutism (excessive hair), tremors
• May cause diabetes, hemolytic-uremic syndrome
• Monitor levels

Question 10

Tacrolimus

Answer 10

• Binds to FK506 which complexes w/ and inhibits calcineurin (greater potency than cyclosporin)
• Side effects similar to cyclosporin, but less likely to cause hyperlipidemia, HT, or hirsutism
• More likely to cause diabetes
• Monitor levels

Question 11

Mycophenolate mofetil (MMF)

Answer 11

• Inhibits monophosphate dehydrogenase (purine synthesis)

- Side effects: GI (diarrhea) and hematologic (leukopenia, anemia)

Question 12

Sirolumus

Answer 12

• Binds to NFKB12 and then inhibits mTOR (blocking signal 3 transduction, from CD25 activation by IL2)
• Side effects: hyperlipidemia, thrombocytopenia, bleeding, proteinuria, pneumonitis
• May decrease the risk of some viral infections (CMV) and neoplasms

Question 13

Rituximab

Answer 13

• Monoclonal Ab that binds to CD20 located on B cells (but not plasma cells)
• B cells may be acting as APCs against the graft
• May have long-term effects and risks

Question 14

Hematopoietic stem cell (HSC) transplant

Answer 14

• Source of HSCs can be BM, peripheral blood, or cord blood

- Same rules apply for organ Tx (autologous, syngenic, or allogenic)

Question 15

Indications for allogenic HSC Tx

Answer 15

• Hematologic malignancies (myeloid and lymphoid): eliminated malignant cells w/ high dose chemo and provide graft-vs-disease (GvD) effect
• BM failure: replace HSCs (aplastic anemia)
• Inherited genetic disease (SCID, SCD): correct genetic defect in cells that originate from HSCs
• Immunodeficiency and autoimmune disease (lupus): reconstitute new immune system

Question 16

HSC (CD34+) mobilization

Answer 16

• HSCs are kept in BM by various proteins binding to BM tissue, namely CXCR4 binding to CXCL12 (SDF1)
• CXCR4 is on the HSC and CXCL12 is released from the niche osteoblast to keep the HSC attached w/in the BM
• Administering GCSF leads to release of proteases by neutrophils that breaks down both CXCL12 and part of CXCR4
• This leads to detachment of the HSCs from the BM and their entry into peripheral circulation

Question 17

Passenger cells

Answer 17

• CD34+ cells from BM are donated along w/ other cells that reside in peripheral blood
• Some of these are CD3+ mature, naive T cells, which are responsible for GvHD (were educated in donor’s thymus)
• However the donated CD34+ HSCs will mature into lymphocytes in the recipients body
• This means the T cells and B cells from the donor HSCs will be educated in the recipient’s thymus and BM, respectively. They will contribute to GvD phenomenon (will not self-react)

Question 18

Types of HSC grafts

Answer 18

• BM aspirates contain high amounts of HSCs, w/ moderate PMNs and platelet recovery time, and a moderate GvHD risk
• Blood stem cells (form GCSF mobilization) contain high amounts of HSCs, w/ low PMNs and platelet recovery time, and a high GvHD risk
• Cord (placental) blood have low HSC content and higher PMN and platelet recovery time, but lowest GvHD risk

Question 19

Graft vs host disease (GvHD) 1

Answer 19

• Donor T cell recognition of genetically disparate recipient who is unable to reject the donor cells
• Donor T cell recognition of host Ags in context of host APCs
• Risk factors: HLA match is biggest (closer the HLA match the better the outcome), also age, sex match, graft source (higher to lower risk: PBSC>BM>cord blood), allosensitization of donor, conditioning intensity

Question 20

Graft vs host disease (GvHD) 2

Answer 20

• Acute: affects skin, liver, GI (usually onset 15-30 days after Tx)
• Maculopapular rash, cholestasis, high AST/ALT, nausea, vomiting, diarrhea
• Chronic: multisystem disease that resembles autoimmune disorders

Question 21

Acute GvHD pathogenesis

Answer 21

• Priming of immune response from conditioning (immunosuppressive chemoRx), pro inflammatory cytokines, activation of APCs
• Induction of T cell activation: recognition by donor T cells of host Ags on APCs in secondary lymphoid tissues, leading to expansion of reactive donor T cells
• Homing of cells to target tissues: mediated by adhesion molecules and chemokines
• Effector phase: destruction of target tissues via cell surface molecules and soluble immune effector molecules

Question 22

Minor histocompatibility Ags

Answer 22

• Peptides derived from polymorphic proteins that differ btwn donor and recipient
• Initiate weaker immune responses than major Ags
• Mediate graft vs disease effect (beneficial)
• In HSC Tx btwn male and female donor/recipient genes on Y chromosome act as minor histocompatibility Ags

Question 23

Graft vs disease effect

Answer 23

• Contributes to cure of malignant disease
• Mediated by expression of tumor Ags, minor and major HLAs on malignant cells
• Extent of effect varies depending on disease: CML>AML>ALL
• Basis for donor lymphocyte infusions

Question 24

Prevention of GvHD

Answer 24

• Post transplant immunosuppression: calcineurin inhibitor (cyclosporine or tacrolimus) w/ another chemo agent (methotrexate or mycophenolate)
• In vitro or in vivo T cell depletion of graft: use of Abs to target donor T cells (ATG), this increases risk for infection

Question 25

Complications of allogenic HSC Tx

Answer 25

• Regimen related toxicity
• Engraftment failure
• Infections (due to neutropenia, catheters, mucosal damage from conditioning and GvHD, steroid and impaired cellular and humoral immunity)
• Impaired cellular immunity due to immunosuppression and acute GvHD
• Impaired humoral immunity due to chronic GvHD
• GvHD (acute and chronic)
• Late effects

Question 26

Viral infections after HSC Tx

Answer 26

• CMV: reactivates upon immune suppression. Seronegative recipients receiving seropositive donor cells are at increased risk (monitored by PCR after HSC Tx)
• EBV
• HSV
• VZV
• HHV6
• BKV
• Respiratory viruses

Question 27

Fungal infections after HSC Tx

Answer 27

• Candida
• Aspergillus
• Other mycoses
• Pneumocytis Jiroveci

Question 28

Conditioning regimen-related toxicity

Answer 28

• Hepatic venocclusive disease: jaundice, fluid retention, tenderhepatomegaly, endothelial injury by toxic drug metabolites
• Idiopathic pneumonia (absence of infectious pathology)
• Diffuse alveolar hemorrhage
• Mucositis
• Hemorrhagic cystitis

Question 29

In utero HSC Tx for SCID

Answer 29

• IL2RG receptor mutation identified at 12 wks gestation
• Results in T cell depleted prenatal BM
• Inject HSC into peritoneal cavity 3x
• Persistent engraftment of donor T cells that were responsive to mitogens and tolerant to recipient