both normal and neoplastic. Vaccines incorporating

such antigens have had little success, perhaps because

the antigens are expressed at some level in normal cells

and tend to induce tolerance that has to be overcome for

induction of effective antitumor immunity.

More recently, there has been interest in developing

personalized cancer vaccines tailored for each patient’s

tumor. As we discussed earlier, the most common antigens that induce immune responses in cancer patients

are neoantigens generated by passenger mutations

affecting random cellular proteins, and the mutations

must be within peptides that can bind to the patient’s

HLA molecules in order to recognized by T cells. A current focus of the tumor vaccination field is to use DNA

sequencing technologies to determine all the mutations

in the protein-coding DNA sequences (exosomes) of

an individual’s cancer cell genome. HLA-binding prediction algorithms are then applied to identify mutant

peptides that are most likely to bind to the HLA alleles

of the patient. After these peptides are defined, personalized tumor vaccines are created using several of the

neoantigen peptides. This approach is promising, but it

also has significant challenges. The vaccines have to be

customized for each patient; effective CTLs have to be

generated by the vaccination (which has been difficult

to do so far with most vaccines, which work by stimulating production of antibodies); tumors may evolve

under the selection pressure of the vaccine-induced

immune response and lose MHC molecules or the target antigens; and because these are therapeutic vaccines

given to tumor-bearing patients, they have to overcome

the immune evasion mechanisms that tumors may have

established in the patient.

Tumor-specific vaccines may be administered as a

mixture of the antigen with adjuvants, just like antimicrobial vaccines. In another approach, a tumor patient’s

dendritic cells are expanded in  vitro from blood precursors, the dendritic cells are exposed to tumor cells or

tumor antigens, and these tumor-antigen–pulsed dendritic cells are used as vaccines. The dendritic cells bearing tumor antigens will theoretically mimic the normal

CHAPTER 10 Immunology of Tumors and Transplantation 207

pathway of cross-presentation and will generate CTLs

against the tumor cells. The success of checkpoint blockade therapies, described previously, has raised hopes that

vaccination used in combination with therapies to block

immune regulation will have added benefits.

Tumors caused by oncogenic viruses can be prevented by vaccinating against these viruses. Two such

vaccines that are proving to be remarkably effective are

against hepatitis B virus (the cause of a form of liver

cancer) and human papillomavirus (the cause of cervical cancer and some types of oropharyngeal cancer).

These are prophylactic vaccines given to individuals

before they are infected, and thus prevent infections by

the tumor-causing viruses.

IMMUNE RESPONSES AGAINST

TRANSPLANTS

Some of the earliest attempts to replace damaged tissues

by transplantation were during World War II as a way

of treating pilots who had received severe skin burns

in airplane crashes. It was soon realized that individuals reject tissue grafts from other individuals. Rejection results from inflammatory reactions that damage

the transplanted tissues. Studies since the 1940s and

1950s established that graft rejection is mediated by the

adaptive immune system because it shows specificity

and memory and it is dependent on lymphocytes (Fig.

10.9). Much of the knowledge about the immunology

of transplantation came from experiments with inbred

strains of rodents, particularly mice. All members of an

inbred strain are genetically identical to one another and

different from the members of other strains. The experimental studies showed that grafts among members of

one inbred strain are accepted and grafts from one strain

to another are rejected, firmly establishing rejection as a

process controlled by the animals’ genes. Later experiments defined the nature of the genes that control graft

rejection and showed that the products of many of these

genes are expressed in all tissues.

As mentioned in Chapter 3, the genes that contributed

the most to the rejection of grafts exchanged between

mice of different inbred strains are called major histocompatibility complex (MHC) genes. The language of

transplantation immunology evolved from the experimental studies. The individual who provides the graft is

called the donor, and the individual in whom the graft

is placed is the recipient or host. Animals that are identical to one another (and grafts exchanged among these

animals) are said to be syngeneic; animals (and grafts)

of one species that differ from other animals of the same

species are said to be allogeneic; and animals (and

grafts) of different species are xenogeneic. Allogeneic

and xenogeneic grafts, also called allografts and xenografts, are always rejected by a recipient with a normal

immune system. The antigens that serve as the targets

of rejection are called alloantigens and xenoantigens,

and the antibodies and T cells that react to these antigens are alloreactive and xenoreactive, respectively. In

the clinical situation, transplants are exchanged between

allogeneic individuals who are members of an outbred

species who differ from one another (except for identical twins). Most of the following discussion focuses on

immune responses to allografts.

Evidence Conclusion

Prior exposure to donor MHC molecules

leads to accelerated graft rejection

The ability to reject a graft rapidly can be

transferred to a naive individual by lymphocytes

from a sensitized individual

Depletion or inactivation of T lymphocytes by drugs

or antibodies results in reduced graft rejection

Graft rejection shows memory

and specificity, two cardinal

features of adaptive immunity

Graft rejection is

mediated by lymphocytes

Graft rejection requires

T lymphocytes

Fig. 10.9 Evidence indicating that the rejection of tissue transplants is an immune reaction. Clinical and

experimental evidence indicates that rejection of grafts is a reaction of the adaptive immune system. MHC,

Major histocompatibility complex.

208 CHAPTER 10 Immunology of Tumors and Transplantation

Transplantation Antigens

The antigens of allografts that serve as the principal

targets of rejection are proteins encoded in the MHC.

Homologous MHC genes and molecules are present

in all mammals; the human MHC is called the human

leukocyte antigen (HLA) complex. It took more than

20 years after the discovery of the MHC to show that

the physiologic function of MHC molecules is to display peptide antigens for recognition by T lymphocytes

(see Chapter 3). Recall that every person expresses

six class I HLA alleles (one allele of HLA-A, -B, and

-C from each parent) and usually six or seven class II

HLA alleles (one allele of HLA-DQ and HLA-DP and

one or two of HLA-DR from each parent). MHC genes

are highly polymorphic, with over 12,000 HLA alleles

among all humans, encoding about 2800 HLA-A proteins, 3500 HLA-B proteins, 2500 HLA-C proteins, 1800

HLA-DRß proteins, 800 DQß proteins, and 700 DPß

proteins. Because of this tremendous polymorphism,

two unrelated individuals are very likely to express several HLA proteins that are different from, and therefore

appear foreign to, each other. Because the genes in the

HLA locus are tightly linked, all the HLA genes from

each parent are inherited together, as a haplotype, in a

Mendelian pattern, and therefore the chance that two

siblings will have the same MHC alleles is 1 in 4.

The reaction to allogeneic MHC antigens on another individual’s cells is one of the strongest immune

responses known. T cell receptors (TCRs) for antigens

have evolved to recognize MHC molecules, which is essential for surveillance of cells harboring infectious microbes.

As a result of positive selection of developing T cells in

the thymus, mature T cells that have some affinity for self

MHC molecules survive, and many of these will have high

affinity for self MHC displaying foreign peptides. Allogeneic MHC molecules containing peptides derived from

the allogeneic cells may look like self MHC molecules

plus bound foreign peptides (Fig. 10.10). Therefore, recognition of allogeneic MHC molecules in allografts is an

example of an immunologic cross-reaction.

There are several reasons why recognition of allogeneic MHC molecules results in strong T cell reactions. Many clones of T cells, including memory T

cells generated from prior infections, that are specific

for different foreign peptides bound to the same self

MHC molecule may cross-react with any one allogeneic MHC molecule, regardless of the bound peptide,

as long as the allogeneic MHC molecule resembles

complexes of self MHC plus foreign peptides. As a

result, many self MHC–restricted T cells specific for

different peptide antigens may recognize any one allogeneic MHC molecule. Also, the process of negative

selection in the thymus eliminates cells that strongly

recognize self MHC, but there is no mechanism for

selectively eliminating T cells whose TCRs have a high

affinity for allogeneic MHC molecules because these

are never present in the thymus. Furthermore, a single allogeneic graft cell will express thousands of MHC

molecules, every one of which may be recognized as

foreign by a graft recipient’s T cells. By contrast, in the

case of an infected cell, only a small fraction of the self

MHC molecules on the cell surface will carry a foreign

microbial peptide recognized by the host’s T cells. The

net result of these features of allorecognition is that the

frequency of alloreactive T cells in any individual is

about 1000-fold greater than the frequency of T cells

that recognize any one microbial antigen.

Although MHC proteins are the major antigens that

stimulate graft rejection, other polymorphic proteins

also may play a role in rejection. Non-MHC antigens

that induce graft rejection are called minor histocompatibility antigens, and most are normal cellular proteins

that differ in sequence between donor and recipient.

These polymorphic proteins yield peptides that are presented by the recipient’s MHC molecules and trigger a T

cell response. The rejection reactions that minor histocompatibility antigens elicit usually are not as strong as

reactions against foreign MHC proteins.

Induction of Immune Responses Against

Transplants

In order to elicit antigraft immune responses, alloantigens from the graft are transported by dendritic cells

to draining lymph nodes, where they are recognized by

alloreactive T cells (Fig. 10.11). The dendritic cells that

present alloantigens also provide costimulators and can

stimulate helper T cells as well as alloreactive CTLs. The

effector T cells that are generated circulate back to the

transplant and mediate rejection.

T cells in allograft recipients may recognize unprocessed donor MHC molecules on the surface of graft

cells, or they may recognize peptides derived from

donor MHC molecules bound to recipient MHC molecules on the surface of recipient APCs (Fig. 10.12).

These two pathways of presentation of graft antigens

have different features and names.

Foreign

peptide

Self MHC molecule presents foreign

peptide to T cell selected to recognize

self MHC weakly, but may recognize

self MHC-foreign peptide complexes well

The self MHC-restricted T cell recognizes

the allogeneic MHC molecule whose

structure resembles a self MHC-foreign

peptide complex

The self MHC-restricted T cell recognizes

a structure formed by both the allogeneic

MHC molecule and the bound peptide

Self

peptide

Self

peptide

Normal

Allorecognition

Allorecognition

A

T cell receptor

Allogeneic MHC

Allogeneic MHC

B

C

Self MHC

Fig. 10.10 Recognition of allogeneic major histocompatibility complex (MHC) molecules by T lymphocytes. Recognition of

allogeneic MHC molecules may be thought of as a cross-reaction in which a T cell specific for a self MHC molecule–foreign peptide complex (A) also recognizes an allogeneic MHC molecule whose structure resembles that of the self MHC molecule–foreign

peptide complex (B and C). Peptides derived from the graft or recipient (labeled self peptide) may not contribute to allorecognition

(B), or they may form part of the complex that the T cell recognizes (C). The type of T cell recognition depicted in B and C is direct

allorecognition.

210 CHAPTER 10 Immunology of Tumors and Transplantation

Recipient

dendritic

cell

Donor

dendritic

cell

Recipient

lymph node

Recipient CD4+

effector cells

Recipient CD8+

effector cells

Allograft

(kidney)

Sensitization

Recipient

effector T cells

Efferent

lymph

vessel

Afferent

lymph

vessel

Blood

Transport of

alloantigens to

lymph node

Activation of T cells, generation

of effector T cells by direct and

indirect antigen presentation

Inflammatory

cytokine secretion

Rejection

Killing of

graft cell

Fig. 10.11 Immune response against transplants. Graft antigens that are expressed on donor dendritic

cells or captured by recipient dendritic cells are transported to peripheral lymphoid organs where alloantigen-specific T cells are activated (the sensitization step). The T cells migrate back into the graft and destroy

graft cells (rejection). Antibodies are also produced against graft antigens and can contribute to rejection (not

shown). The example shown is that of a kidney graft, but the same general principles apply to all organ grafts.

• Direct allorecognition. Most tissues contain dendritic cells, and when the tissues are transplanted, the

dendritic cells in the graft may migrate to secondary

lymphoid organs of the recipient. When naïve T cells

in the recipient recognize donor allogeneic MHC

molecules on these graft-derived dendritic cells,

the T cells are activated; this process is called direct

recognition (or direct presentation) of alloantigens.

Direct recognition stimulates the development of

alloreactive T cells (e.g., CTLs) that can then directly

recognize the allogeneic MHC molecules on cells of

the graft and destroy the graft.

• Indirect allorecognition. Graft cells (or alloantigens) may be ingested by recipient dendritic cells

and transported to draining lymph nodes. Here,

donor alloantigens are processed and presented by

self MHC molecules on the recipient APCs. This

process is called indirect recognition (or indirect

presentation) and is similar to the cross-presentation

of tumor antigens to CD8+ T cells, discussed earlier.

If alloreactive CTLs are induced by the indirect pathway, these CTLs are specific for donor alloantigens

displayed by the recipient’s self MHC molecules on

the recipient’s APCs, so they cannot recognize and

kill cells in the graft (which, of course, express donor

MHC molecules). When graft alloantigens are recognized by the indirect pathway, the subsequent

rejection of the graft likely is mediated mainly by

alloreactive CD4+ T cells. These T cells may enter

the graft together with host APCs, recognize graft

antigens that are picked up and displayed by those

APCs, and secrete cytokines that injure the graft by

an inflammatory reaction. Indirect allorecognition

by host CD4+ T cells also contributes to stimulating

production of host antibodies that bind to graft MHC

molecules, as discussed later.

CHAPTER 10 Immunology of Tumors and Transplantation 211

We do not know the relative importance of the direct

and indirect pathways of allorecognition in T cellmediated rejection of allografts. The direct pathway

may be most important for CTL-mediated acute rejection, and the indirect pathway may play a greater role in

chronic rejection, as described later.

T cell responses to allografts require costimulation,

but which stimuli in grafts enhance the expression of

costimulators on APCs is unclear. As with tumors, graft

cells may undergo necrosis, perhaps in the period of

ischemia before the transplant is done, and substances

released from the injured and dead cells activate APCs

by innate immune mechanisms. As we discuss later,

blocking costimulation is one therapeutic strategy for

promoting graft survival.

The mixed lymphocyte reaction (MLR) is an in vitro

model of T cell recognition of alloantigens. In this model,

T cells from one individual are cultured with leukocytes

of another individual, and the responses of the T cells are

assayed. The magnitude of this response is proportional

to the extent of the MHC differences between these individuals and is a rough predictor of the outcomes of grafts

exchanged between these individuals.

Although much of the emphasis on allograft rejection has been on the role of T cells, it is clear that alloantibodies also contribute to rejection. Most of these

antibodies are helper T cell–dependent high-affinity

antibodies. In order to produce alloantibodies, recipient

B cells recognize donor alloantigens and then process

and present peptides derived from these antigens to

helper T cells (that may have been previously activated

by recipient dendritic cells presenting the same donor

alloantigen), thus initiating the process of antibody production. This is a good example of indirect presentation

of alloantigens, in this case by B lymphocytes.

Immune Mechanisms of Graft Rejection

Graft rejection is classified into hyperacute, acute, and

chronic, on the basis of clinical and pathologic features

(Fig. 10.13). This historical classification was devised by

clinicians based on rejection of kidney allografts, and it

has stood the test of time remarkably well. It also has

become apparent that each type of rejection is mediated

by a particular type of immune response.

• Hyperacute rejection occurs within minutes of transplantation and is characterized by thrombosis of graft

Allogeneic MHC

Alloreactive

T cell

Alloreactive

T cell

Self

MHC

Dendritic cell

in recipient

Allogeneic

MHC

Uptake and

processing of

allogeneic MHC

molecules by

recipient APC

Direct allorecognition

Indirect allorecognition

Allogeneic

antigenpresenting cell

(dendritic cell)

Allogeneic

tissue cell

Peptide derived

from allogeneic

MHC molecule

T cell recognizes

processed

peptide of

allogeneic MHC

molecule bound

to self MHC

molecule

on host APC

T cell recognizes

unprocessed

allogeneic

MHC molecule

on graft APCs

A

B

Fig. 10.12 Direct and indirect recognition of alloantigens. A, Direct alloantigen recognition occurs when T

cells bind directly to intact allogeneic major histocompatibility complex (MHC) molecules on antigen-presenting cells (APCs) in a graft, as illustrated in Fig. 10.8. B, Indirect alloantigen recognition occurs when allogeneic

MHC molecules from graft cells are taken up and processed by recipient APCs, and peptide fragments of

the allogeneic MHC molecules are presented by recipient (self) MHC molecules. Recipient APCs also may

process and present graft proteins other than allogeneic MHC molecules.

212 CHAPTER 10 Immunology of Tumors and Transplantation

vessels and ischemic necrosis of the graft. Hyperacute

rejection is mediated by circulating antibodies that are

specific for antigens on graft endothelial cells and that

are present before transplantation. These preformed

antibodies may be natural IgM antibodies specific for

blood group antigens (discussed later in this chapter), or

they may be antibodies specific for allogeneic MHC molecules that were induced by previous exposure to allogeneic cells due to blood transfusions, pregnancy, or prior

organ transplantation. Almost immediately after transplantation, the antibodies bind to antigens on the graft

vascular endothelium and activate the complement and

clotting systems, leading to injury to the endothelium

and thrombus formation. Hyperacute rejection is not a

common problem in clinical transplantation, because

every donor and recipient are matched for blood type

and potential recipients are tested for antibodies against

the cells of the prospective donor. (The test for antibodies

is called a cross-match.) However, hyperacute rejection is

a major barrier to xenotransplantation, as discussed later.

• Acute rejection occurs within days or weeks after

transplantation and is the principal cause of early graft

failure. Acute rejection is mediated by T cells and antibodies specific for alloantigens in the graft. The T cells

may be CD8+ CTLs that directly destroy graft cells or

CD4+ cells that secrete cytokines and induce inflammation, which destroys the graft. T cells may also react

against cells in graft vessels, leading to vascular damage.

Antibodies contribute especially to the vascular component of acute rejection. Antibody-mediated injury to

graft vessels is caused mainly by complement activation

by the classical pathway. Current immunosuppressive

therapy is designed to prevent and reduce acute rejection by blocking the activation of alloreactive T cells.

• Chronic rejection is an indolent form of graft damage that occurs over months or years, leading to progressive loss of graft function. Chronic rejection may

be manifested as fibrosis of the graft and by gradual

narrowing of graft blood vessels, called graft arteriosclerosis. In both lesions, the culprits are believed to be

T cells that react against graft alloantigens and secrete

cytokines, which stimulate the proliferation and activities of fibroblasts and vascular smooth muscle cells

in the graft. Alloantibodies may also contribute to

chronic rejection. Although treatments to prevent or

curtail acute rejection have steadily improved, leading to better 1-year survival of transplants, chronic

rejection is refractory to most of these therapies and is

becoming the principal cause of graft failure.

Prevention and Treatment of Graft Rejection

The mainstay of preventing and treating the rejection

of organ transplants is immunosuppression, using

drugs that deplete T cells or inhibit T cell activation

and effector functions (Fig. 10.14). The development

of immunosuppressive drugs launched the modern era

of organ transplantation because these drugs made it

feasible to transplant organs from donors that were not

HLA-matched with recipients, especially in situations

when such matching was impractical, such as transplantation of heart, lung, and liver.

One of the first and still most useful classes of immunosuppressive drugs used in clinical transplantation are

the calcineurin inhibitors, including cyclosporine and

tacrolimus (FK506), which function by blocking the protein phosphatase calcineurin. This enzyme is required

to activate the transcription factor NFAT (nuclear factor of activated T cells), and blocking its activity inhibits the transcription of cytokine genes in the T cells.

Another widely used drug is rapamycin, which inhibits

a kinase called mTOR (mammalian target of rapamycin)

required for T cell activation. Many other immunosuppressive agents are now used as adjuncts to or instead of

calcineurin and mTOR inhibitors (see Fig. 10.14).

All of these immunosuppressive drugs carry the problem of nonspecific immunosuppression (i.e., the drugs

inhibit responses to more than the graft). Therefore,

patients receiving these drugs as part of their post-transplantation treatment regimen become susceptible to

infections, particularly by intracellular microbes, and the

patients have an increased risk of developing cancers, especially skin cancers and others caused by oncogenic viruses.

The matching of donor and recipient HLA alleles by

tissue typing had an important role in minimizing graft

rejection before cyclosporine became available for clinical

use. Although MHC matching is critical for the success

of transplantation of some types of tissues (e.g., hematopoietic stem cell transplants) and improves survival of

other types of organ grafts (e.g., renal allografts), modern

immunosuppression is so effective that HLA matching is

not considered necessary for many types of organ transplants (e.g., heart and liver), mainly because the number

of donors is limited and the recipients often are too sick to

wait for well-matched organs to become available.

The long-term goal of transplant immunologists is to

induce immunological tolerance specifically for the graft

alloantigens. If this is achieved, it will allow graft acceptance without shutting off other immune responses in

the host. However, many years of experimental and

C