Renal Transplant Immunobiology

Question 1

Both cytotoxic T lymphocytes (CTLs) and Ab-producing B cells are important in mediating rejection.

Answer 1

That’s true. These both get activated by T helper cells.

Question 2

Why is it significant that HLA genes are located very close to each other in the genome?

Answer 2

They’re inherited via haplotypes without crossing over between the HLA genes. Siblings have a 1/4 chance of having exactly matched HLA genotypes.

Question 3

Which HLA loci are actually matched when matching solid organs?

Answer 3

HLA-A, HLA-B, and HLA-DR.

I’m not sure why… Probably just because DR is so close to DP and DQ that they’re very tightly linked?

Question 4

Syngeneic, allogeneic, and xenogeneic?

Answer 4

Syngeneic - identical twins. (I don’t think this term applies if just the HLA genotype is the same… but not 100% sure)
Allogeneic - same species, different genes.
Xenogeneic - Different species.

Question 5

The probability that T cells recognize a given peptide antigen is 1/100,000 to 1/1,000,000. What’s the probability that a given T cell will recognize an alloantigen?

Answer 5

1/20
(Which, you should recognize is astoundingly high. Maybe this is because of the way T cells develop - selected for loose affinity for self-MHC + self-peptide; if there’s slightly different donor-MHC or donor-peptide, they’re able to react? (I’m making this up… but it helps me make sense of this.)).

Question 6

What is direct allorecognition?

Answer 6

A donor antigen-presenting cell (APC) presents donor-MHC + donor-peptide to a recipient T cell.
(the slide just says APC, but the way he talks about cells migrating out of the tissue, it sounds like he’s really talking about pAPCs like DCs and macrophages)

Question 7

What is indirect allorecognition?

Answer 7

A recipient pAPC migrates into the graft, picks up some some donor antigen and present it on recipient-MHC to a recipient T cell.
(Non-self Ag on self-MHC to self-T cell)

Question 8

Recall that this whole presentation to helper T cells business mainly happens in the lymph nodes.

Answer 8

OK

Question 9

What is time frame over which direct vs. indirect alloresponses to a graft will happen?
(How does this correspond to the graft survival curves?)

Answer 9

Direct: Greatest immediately post-transplant, but then dwindles as the donor (p)APCs turn over and aren’t replaced with new ones from bone marrow.

Indirect: Starts low, but increases with time as more recipient pAPCs have had time to migrate into the graft tissue and present Ag to recipient T cells.

(explains the shape of the survival curves of transplanted grafts: higher rate of rejection early on, with slower rates of rejection as direct recognition as petered out.)

Question 10

What mediates hyperacute rejection?
Time frame?
What can be done to prevent it?

Answer 10

Hyperacute rejection is from pre-formed Abs against MHC I molecules (from prior sensitization) or blood-group (ABO) antigens.
This happens in minutes - can see kidney turn black in the OR.
Prevention: Test for Abs, match blood type. If necessary, can try to desensitize the recipient.

Question 11

Time frame for acute humoral and acute cellular rejection?

How are these prevented (broadly speaking)?

Answer 11

Time frame: Days to weeks.

Prevent/treatment: Immunosuppression.

Question 12

What mediates “chronic rejection”?

Answer 12

It’s a mix of immune and non-immune causes.
Non-immune causes include…
-ischemic injury
-calcineurin inhibitors

Question 13

What does H&E of a kidney that experienced hyperacute rejection look like?

Answer 13

Lots of necrosis. Some thrombus in the capillaries.

Not much cellular infiltrate- there wasn’t time for it.

Question 14

What does H&E of a kidney with acute cellular rejection look like?

Answer 14

Mononuclear infiltrate
- tubulitis and interstitial infiltrate.
(as you’d expect, lots of lymphocytes)

Question 15

What does H&E of a kidney with acute humoral rejection look like?

Answer 15

Vasculitis. I
Interstitial hemorrhage.
Capillary-endothelial swelling.
(there isn’t necessarily a lymphocytic infiltrate, but there can be)

Question 16

What are 2 ways to more specifically distinguish between acute cellular and acute humoral rejection?

Answer 16

Acute humoral rejection will have:

• Circulating anti-donor Abs.
• C4d deposits in the tissue.

Question 17

What’s specially about C4d?

Answer 17

It is deposited into the tissue where complement is activated. (in this case, the complement is activated via antibodies)

Question 18

Typical histologic features of chronic rejection?

Answer 18

Fibrosis and neointimal thickening.

Question 19

3 effects of immunosupression?

Which effect limits their usefulness?

Answer 19

Immunosuppression.
Immuno-deficient complications.
Non-immune toxicity (this tends to be the limiting factor).

Question 20

Immuno review: 3 signals required for full activation of a T cell?

Answer 20

Signal 1: TCR binding MHC + Ag. (from APC)
Signal 2: CD28 binding B7. (from APC)
Signal 3: IL-2R binding IL-2. (autocrine/paracrine)

(I guess other cytokines could be considered part of Signal 3, as well.)

Question 21

How can Signal 1 (TCR binding) be blocked from outside the cell? (2 ways)

Answer 21

Thymoglobulin - polyclonal anti-TCR Abs.

Monoclonal anti-CD3 (OKT3, or newer chimeric/humanized versions).

Question 22

What second messenger is used to conduct TCR signaling?

What are the downstream effects of this?

Answer 22

Increased intracellular Ca++.
Ca++ binds calcineurin (important!).
Activated calcineurin allows NFAT (a transcription factor) to translocate into the nucleus (less important to know).

Question 23

What are 2 drugs that inhibit intracellular TCR signaling? What are their molecular targets?

Answer 23

Cyclosporin binds cyclophilin.
Tacrolimus (FK506) binds FKBP12.
Each of these create a complex that inhibits calcineurin.
(tacrolimus is preferred, I believe)

Question 24

What’s a drug that targets Signal 2?

Answer 24

Belatacept is CTLA-4-Ig, which binds B7.1 and B7.2, preventing them from binding CD28. (This is soluble CTLA-4. This is NOT an anti-CTLA-4 Ab.)

(recall that drugs that inhibit CTLA-4 have been used to try to “unleash” the immune system against things like melanoma)

Question 25

What process happens to T cells that get TCR signaling but no 2nd signal (CD28:B7)?

Answer 25

Anergy.

this is a natural mechanism of “peripheral tolerance”

Question 26

Can you make antibodies that antagonize IL-2R?

Answer 26

Sure. They bind to a part of it that’s called CD25… but I wouldn’t worry about that too much.

Question 27

2 ways to block the intracellular transduction of Signal 3?

Answer 27

Inhibit mTOR.

Inhibit Jak3.

Question 28

What are 2 drugs that inhibit mTOR?
What is the molecular target (of at least one of them..)?
(What does mTOR stand for?)

Answer 28

mTOR = mammalian target of rapamycin.
Rapamycin (aka. sirolimus) and everolimus inhibit mTOR.

Both these drugs bind FKBP12, making a complex that inhibits mTOR - and not calcineurin. (Interesting, FKBP12 can inhibit different molecules depending on which drug binds it.)

Question 29

Immuno review: 3 mechanisms of tolerance?

Answer 29

Central: Deletion of autoreactive cells during development. (but… unless you did something to the thymus, it’s not really relevant).
Peripheral:
-Anergy - Signal 1 without Signal 2.
-Tregs - T lymphocytes that dampen immune response.

Question 30

What’s one method that’s been used to induce tolerance to a kidney transplant?

Answer 30

Simultaneous bone marrow / HSC transplant from same donor.
(I’m trying to think about why this works. Maybe because donor-type pAPCs generated from the bone marrow transplant tolerize T cells to donor MHC +/- donor Ags… in the thymus and elsewhere?)

Question 31

How do you measure tolerance? Wouldn’t it be useful to know if someone who has had a kidney for 20 years could come off immunosuppressives?

Answer 31

It’s a work in progress… but there’s no definitive way to do it yet.
(some B cell signals came up in some patients that had developed tolerance to a donated kidney)