Immunology of the intestinal tract

GASTROENTEROLOGY

1993;105:1275-1280

Immunology of the Intestinal Tract MARTIN

F. KAGNOFF

Department

of Medicine, University of California, San Diego, La Jolla, California

T

he past half century age” of mucosal

pline

within

have

been

diversity,

immunology

gastroenterology. made

tinal immune

of intestinal

disease

tinal immunology

have evolved.

in the

and molecular

increased

mune

system

and

techniques,

techniques,

raflow

and polymerase

on one reader’s

our

knowledge have, in turn,

by many

immunology

who,

have contributed Specialized

different

groups

individually

from the systemic draw on these ment of human ogy an attractive basic scientists.

that

immune

im-

A number

of

laid the background studying

distinguish

discovery

intes-

by Chodirker

immunoglobulin

A (IgA)

immunoglobulin

in intestinal

the major advances credited

and Tomasi

is the predominant secretions’

with moving

forward

the entire

immunology.

Shortly

thereafter,

tions

was shown

to differ

structurally

spects

from

unlike

most serum

ultimately, peptide

IgA in serum.’

class of

to contain

Together,

and can be field of intes-

IgA

in secre-

in several

re-

IgA in secretions,

IgA, was shown

was shown

chains.

Thus,

that

was one of

of the past half century

tinal

to be dimeric

and,

two additional

this complex

poly-

was termed

se-

polypeptide cretory IgA (sIgA). 0 ne of the additional chains in sIgA initially was termed “transport piece,” it was proposed

IgA transport,

a notion

to be correct.

Shortly

renamed

“secretory

main

to play an important that subsequently thereafter,

brane

surface

protein

the binding,

protein

of epithelial

serves uptake,

piece

was

(SC). Subsequently,

to be the cleaved

of a transmembrane

role in

was proven

transport

component”

SC was recognized basolateral

and collectively,

to this field’s dynamic features

have substan-

of the intestinal

The

because

view of seminal

that

over the past half century.

these contributions for advances

and immunoge-

immunoabsorbent

and key developments

tially

tinal

of immunochemistry,

technology).

This essay focuses discoveries

in intes-

fluorescence-activated

cell separation

reaction

that

in part, by concur-

antibody

M~cosal

SUrfSCeS

sys-

of new methods

A: The

hMUtR00~lin

Produced at Inte8tksi

interactions

Advances

enzyme-linked

immunohistology,

cytometry,

immune

and parasites

immunology,

(e.g., monoclonal

dioimmunoassays,

chain

fields

and by the development

techniques

disci-

and in the pathogenesis

have been fueled,

discoveries

assays,

viruses,

surfaces

PrOdOml08nt

advances

into the role of the intes-

bacteria,

lmmunoglobulin

of

the organization,

system in host-environment

take place at mucosal

netics,

as a major

of the intestinal

new insights

with food antigens,

cellular

the “coming

Remarkable

in understanding

and function

tem. In parallel,

rent

has witnessed

extracellular expressed

doon the

cells. This transmem-

as an epithelial and transport

cell receptor

for

of the IgA dimer

growth.

across

the

The transmembrane protein, when present on the cell surface, is termed the “polyimmunoglobulin receptor” (pIgR) and, w h en p resent in its cleaved form as part of the sIgA molecule, is referred to as SC. The second

intestinal

system and the potential

features for the prevention disease have made intestinal

to

and treatimmunol-

area of study for both clinicians Thus, ongoing multidisciplinary

and re-

search and advances are leading to novel approaches in the development of mucosal vaccines and to new and more effective methods for the diagnosis and treatment of intestinal disease.

epithelial

cells and

into

the intestinal

lumen.

polypeptide chain associated with dimeric IgA was described by Halpern and Koshland,3 and Mestecky et a1.4 and is termed “J chain.” J chain is expressed by the 0 1993 by the American Gastroenterologlcal 0016-5085/93/$3.00

Association

1276

MARTIN

same

lymphocytes

mucosa

F. KAGNOFF

GASTROENTEROLOGY

and plasma

that synthesize

the IgA molecule

to play a role in IgA polymerization. nized

that the pIgR

into intestinal

mans

and Heremans

constant Most

region

IgA

in serum

is IgAl.

in greater

being

higher

gut.

Structural

lower IgA2

determine

Important

colonization other

IgA,

in several

classes

unlike

classes, is relatively pate

to a large extent,

the

has also been other

nonphlogistic

that are readily activated

globulin

classes (e.g., complement

directed

cytotoxic

harmless

responses).

nonpathogenic

quantities, it makes

within sense

tigen binding tory regard

responses

that rather

to

impor-

cytotoxic

re-

by the other immunoactivation,

Given

the broad array of moieties

pensated tion

As an “experiment

in selective because

experience

the spectrum

vascular

incidence

teins

in the gut by IgA and ensuing

components

by decreased these

or, alternatively,

ity in immunoregulation netic

background

ciency.

As

shown

by

the association different

HLA

haplotype

diseases

and

an underlying

Crabbe

of pro-

immunological

proteins

results

dis-

disorders.

elimination

associated

that

produc-

IgA deficiency

and allergic

may be caused

between

can be com-

of autoimmune

disease,

This

cross-reactions

of clinical

by increased with

of

is quite vari-

IgA levels

individuals

an increased

be expected function

IgA deficiency

individuals

Some

of nature,”

would

physiological

decreased

for in many

of IgM.

host

cell

abnormal-

with the same ge-

in selective and

with several

IgA

Heremans,

defiIgA

other diseases,

disease. * This association may reflect of several different diseases with sev-

genes

that

rather

that is caused

are encoded

than

on a common

a predisposition

by the deficiency

to these

in IgA.

Gut-Associated Lymphoid Tissue, “M” Cells, and the “Common Mucosal Immune System”

an is an-

of inflamma-

Abnormalities such

immu-

IgA in the gut. Nonetheless,

surface,

produces

function

the activation

is the most common

normally

mucosal tract

It

epithe-

to shed light on the normal

eral

antibody

lumen.

agents within

IgA predictably

is also associated

major effector

in diseases

lacking

celiac

or damaged

in the mucosa.

in whites.

individuals

including

the intestinal than

infectious

deficiency

and

complexes

across the epithe-

back into the intestinal

and does not partici-

antigenic

whose

are evident

More-

immune

of

on the

also that IgA may play a role in host

IgA deficiency

nodeficiency

the gut that can, at least in small

cross normal

immunoglobulin

viral

manner,

up and transported

immunoglobulin

proinflammatory

sponses

encountered

surfaces.

by

of what IgA does not do relative

immunoglobulin Thus,

of its strucand

Selective

ease, collagen

IgA function

to the pIgR

lial cells.

able, likely

IgA2 form be-

at mucosal

In this

No. 5

layer after binding

IgA dimer

by neutralizing

and

IgA pro-

bacterial

defense

abnormalities

produced

preventing

and invasion

over, the discovery

IgAl

in the delineation

cell. be taken

the

of these

in understanding

and

with

to

by these enzymes.

its function,

antigens

in dif-

sensitivities

predominant

advances

IgA mediates

binding

compared

to degradation

advances

have paralleled

tant.

but varies

the ratio of IgAl

to bacterially

resistance

tract,

with

between

would

~1 14.

quantities

differences

teases, with the intestinal ing more

and

by two different

the epithelial

lial cell and secreted

IgA in hu-

In the intestinal

the differential

epithelial

has been suggested

and IgA2.5 IgAl

in the upper

two IgA subclasses

ture.

IgAl

that

to be encoded

parts of the intestine

IgA2

is an integral

heavy chain genes on chromosome

IgA2 is present ferent

IgA and IgM

IgA and IgM. reported

exists in two forms,

IgA2 are now known

and appears

and that J chain

of both polymeric

Vaerman

both

through

the antigen-associated

It is now recog-

can transport

secretions

component

eliminated

cells in the intestinal

Vol. 105,

in this

as inflammatory

Peyer’s extend

through

patches the

are lymphoid mucosa

and

aggregates submucosa

that of the

bowel disease and celiac disease, in which acute and chronic mucosal inflammation is characterized by a relative increase in IgG-producing cells (i.e., activates inflammatory responses) compared with IgA produc-

small intestine. Other isolated lymphoid follicles are also present in the small intestine, and lymphoid aggregates are present in the rectum and colon. Early studies suggested that Peyer’s patches in the small intestine were the primary site of B cell differentiation in

ing cells.6 IgA is generally thought to act mainly at the luminal mucosal surface. However, recent data by Kaetzel et al.’ suggest that dimeric IgA may also function on the basolateral side of intestinal epithelial cells by binding antigens that have penetrated the epithelial cell layer. Such IgA-antigen immune complexes would then be

humans (until the 197Os, the prevailing notion that T cells developed in the thymus, whereas B developed in Peyer’s patches). It is now known the primary site of B-cell development is the fetal and bone marrow. The recognition by Craig and bra’ that Peyer’s patches are a major site where B ultimately become committed to the IgA class and

was cells that liver Cecells that

November

1993

Peyer’s patch B cells ultimately leave Peyer’s patches and populate the lamina propria of the intestinal tract provided a new conceptual framework for mucosal immunology. T cells that populate the lamina propria and intraepithelial region of the intestine also appear to derive, at least in part, from Peyer’s patches. The above discoveries led to the notion of a “common mucosal immune system.” It is now known that lymphocytes primed by antigens in intestinal mucosal lymphoid follicles exit these sites and undergo a migratory pathway. This pathway includes transit through the lymphatics and mesenteric lymph nodes, entry into the systemic circulation, and subsequently “homing” back to the intestinal lamina propria as well as to other mucosal sites in the respiratory tract, the female genital tract, and the female breast during pregnancy and lactation. Based on this concept, enteric immunization predictably should result in host protection at other mucosal sites, including the respiratory and female genital tracts, and in protective antibody in breast milk that mirrors the mother’s immune response to antigens in the gut. Such appears to be the case, although the common mucosal immune system may have more regional specialization than initially appreciated. For example, immunization of the lower gastrointestinal tract may lead to greater concurrent mucosal immunity in the female genital tract than immunization of the upper intestinal tract. Lymphocyte migration between varying sites within the mucosal immune system and lymphocyte migration between the systemic immune system and mucosal immune system was recognized several decades ago. However, insight into the molecular and biochemical basis underlying this migration has emerged mostly over the past decade. Thus, homing and adhesion molecules on lymphocytes, and reciprocal ligands (addressins) in mucosal lymphoid tissues, provide a “lock and key” and binding system for the selective migration and retention of lymphocytes within selected mucosal and systemic sites. Discovery of the “M cell” represents another important contribution that has increased understanding of the intestinal immune system. Initial studies by Bockman and Cooper” and subsequent studies by Owen and Jones” documented the presence of specialized epithelial cells overlying Peyer’s patches and other lymphoid aggregates at mucosal sites. These cells, termed M cells, are thought to be the major site of antigen entry into mucosal lymphoid follicles. This is especially the case for particulate antigens. In contrast, many enteric pathogens enter the host directly follow-

IMMUNOLOGY

OF THE INTESTINAL

TRACT

1277

ing infection of epithelial cells, other than M cells, or following invasion between epithelial cells.

Mucosal T=CeH Popuiaths Cell-Mediated Immunity

and

Characterization of the lymphoid populations located between intestinal epithelial cells (intraepithelial lymphocytes [IEL]) and those in the lamina propria has made remarkable advances over the past decades. A number of investigators have shown that IEL in humans are predominantly T lymphocytes. The majority of these T cells have the CD8 phenotype and carry the a/p form of the T-cell receptor for antigen. IEL have specialized surface molecules that appear to govern their homing or anchoring within the intraepithelial region. A minor fraction of human IEL T cells (5%10%) carry the more recently described y/S form of the T-cell receptor.12 Although a number of groups have reported that the ratio of y/6 to cl/p IEL in the small intestine is increased in celiac disease in contrast to other small intestinal diseases, the role these cells play in the pathogenesis of celiac disease is still unknown. Current concepts envision that y/6 cells play a major role in early host mucosal defense to pathogens or perhaps react to antigens expressed on stressed epithelial cells. The specific antigens that y/S T cells recognize and the putative host restriction elements they use are not yet known and are a subject of intense study. Nonetheless, it is known that cr./p and y/s IEL can produce a broad array of cytokines and can show cytolytic activity, suggesting they play an important role in host mucosal cell mediated immunity. In contrast to IEL T cells, a majority of T cells in the lamina propria (-two thirds) have the CD4 phenotype usually associated with HLA class II restricted T cells, carry the a//3 T-cell receptor and are thought to mediate helper function for antibody and T cellmediated responses. These cells also provide signals that alter the activation and function of other cell types in the lamina propria (e.g., monocyte/macrophages, mast cells, basophils, eosinophils). The CD4+ T-cell population in the lamina propria plays a major role in the pathogenesis of celiac disease. The importance of the lamina propria a/p CD4+ T-cell population is highlighted also by the immunodeficiency syndromes. For example, as shown by Rodgers et al., during the course of human immunodeficiency virus 1 infection, there is a marked decrease in the mucosal CD4 T-cell population, a reversal of the normal CD4/

1278

MARTIN

GASTROENTEROLOGY

F. KAGNOFF

CD8 ratio, and the development nistic mucosal infections.13

of numerous opportu-

Cytoklnes Cytokines are small molecular weight, biologically active mediators. They comprise the language by which lymphocytes and other cells “talk to each other,” and deliver activation, growth, and differentiation signals to themselves and surrounding cells in the intestinal mucosa. The discovery and detailed characterization of a wide range of different cytokines (e.g., interleukins [ILs] l-1 3, interferons, transforming growth factor ps [TGFPs]) and, in some cases, the elucidation of their effects on cell types present within the intestinal mucosa has led to important new concepts and advances in intestinal immunology. Lymphocytes, monocyte/macrophages, mast cells, basophils, and eosinophils within the intestinal mucosa communicate not only with each other but also with intestinal epithelial cells through the release of cytokines. Some cytokines can alter the electrolyte secreting properties of epithelial cells, mucosal vascular permeability and, together with other proinflammatory mediators produced in the intestinal mucosa (e.g., prostaglandins, leukotrienes, kinins, platelet activating factor), play a key role in the generation of the acute and chronic inflammatory response in the intestinal mucosa. Recent observations that intestinal epithelial cells themselves produce cytokines such as IL-8 in a regulated manner and that such mediators in turn provide important activating and chemotactic signals to other cell types in the underlying mucosa are providing new insights into how bacterial and viral invasion of the epithelium may signal the activation of the acute intestinal inflammatory response within the gut.i4,i5 Other mucosal cytokines are now recognized to play a major role in antigen driven antibody and cell mediated immune responses and in immunoglobulin heavy chain Thus, as shown initially in animal class “switching.” studies and later in humans, IL-4 plays a role in the switch of B cells into the IgE class, whereas TGFPl plays a role in the switch of B cells into the IgA class.” Cytokines such as IL-2, IL-4, and IL-6 also markedly influence human IgA B cell differentiation, and IL-5 is a potent activator and chemoattractant for eosinophils during parasitic infection.

Oral Tolerance Oral tolerance is defined as the decreased ability to stimulate a systemic immune response to antigens previously encountered in the gut.” The initial discov-

Vol. 105.

No. 5

ery of this phenomenon by Chasei represents another seminal advance which, although largely ignored for a number of years, has substantially increased understanding of the intestinal immune system. The intestinal mucosa normally contacts many different food, bacterial, viral, parasitic, and chemical products. Moreover, proteins in small quantities can gain access to the intestinal mucosa across or between epithelial cells and to the systemic circulation from the normal and damaged intestinal tract. Although not nutritionally significant, these proteins may be sufficient to provide an antigenic stimulus. The notion that there would be no advantage, and a great potential disadvantage, if the host were to respond systemically to multiple “harmless” antigens that cross mucosal surface, led to the hypothesis that exposure to antigens in the intestinal tract might, in fact, stimulate downregulation of systemic immune responses. Experimental data showed this was the case in animal species, and this also appears to be the case in humans. The mechanisms responsible for the development of oral tolerance are a subject of active study and, as suggested by Miller et al., may involve the release of cytokines such as TGFPl that down-regulate the immune response.” Possible abnormalities in the development and maintenance of “oral tolerance” have been proposed to underlie the development of several systemic autoimmune diseases (i.e., the host is envisioned to develop immune responses to gut antigens that crossreact with self antigens). Moreover, the ability to increase oral tolerance is being tested as the basis for the prevention and treatment of several autoimmune diseases. Approaches to the development of mucosal vaccines delivered via the enteric route and delivery systems for such vaccines also have been substantially influenced by knowledge of the existence of this phenomenon. In contrast to most soluble antigens and particulate antigens that appear to enter the host via mucosal follicles, oral tolerance does not develop to antigens that persist and replicate within the intestinal mucosa or to pathogenic viruses and bacteria that directly invade across mucosal surfaces. For the latter, a host systemic immune response develops and often is necessary for host survival.

Mucosal Vaccines Many groups are currently working on the development of enteric vaccines for the prevention of a broad array of intestinal and systemic infections. Recent advances in this area have been possible largely

November

IMMUNOLOGY

1993

because

of a clearer understanding

ology of the intestinal M cells, lymphocyte tures

immune migration

of the sIgA system,

ance).

Antigens

and viable been

that

antigens

cines are using

spheres, antigens,

biodegradable

sorbed

antigens,

tigen

currently

to

under

decade

should

Peyer’s

active witness

new information delivery

systems

surface

and protocols

target an-

cells

also

Thus,

optimal

abare

the next

of substantial mucosal

for mucosal

vaccine

immuniza-

tant

in the

role

IBD.

cytokine

Disease

nists)

Following the discovery that highly polymorphic genes in the HLA region play a key role in transplantation

rejection

response, tions

numerous

between

and in regulation groups

intestinal

otal in area was the initial that celiac

disease

markers.20

Others

chronic

active

recognition

was strongly showed

hepatitis,

tis, and selective

associated

associations primary

IgA deficiency

the HLA

class I and class II genes,

contains

It is now known numerous

factor,

heat shock

large multifunctional intracellular

in the

proteins transport

genes

such as tumor

reticulum

proteasome

degradation

(TAP

cholangito

including necrosis

components

of a impor-

of cytoplasmic

(LMP genes), and proteins important of cellular peptides from the cytosol

endoplasmic

the role of the intestinal

this chromosomal

or stress proteins,

tant

HLA

that in addition

additional

cytoplasmic

immunology,

with genes that map to

region.

that code for cytokines

with

for the into the

genes).

have an impor-

concepts

response

in

regarding

the inflammatory

the

process

in

to new classes of drugs (e.g., cytokine

receptor

as potent

antago-

anti-inflammatory

with

intestinal

physiology

its rapid

growth

growth studies

in normal of such

of new approaches of disease.

into

and ultimately

for autoimmune

of the

for

For example,

have led to new insights

that Peyer’s patches initiation

about

field is still in

spin-off

of autoimmunity

and IgA responses

gen enters

system This

A major

and treatment

mechanisms

site for

sponse, basis

immune

lead to new treatments

The discovery

of knowledge

and disease.

of oral tolerance

possible

tive

an explosion

phase.

the birth,

of a new field, intestinal

will be the development

the diagnosis

could

has witnessed

and early adolescence

are a major

mucosal

in particular,

disease. induc-

immune

re-

and that anti-

these sites via the M cell, forms one current

for the development

and vaccine

Celiac disease has become a model system for exploring the relationship between HLA genes and in-

mediators

mucosa

last half century

et al.

of autoimmune

sclerosing

the HLA region

The infancy,

by Falchuk

that proin-

soluble

inflammatory

and

Piv-

genes.

of IBD

Summary

of the immune

and HLA

agent(s)

suggest

emerging

can be tested

disease

other

ongoing

are leading

mu-

bowel

in IBD.

began to search for associa-

disease

and envi-

susceptibility.

studies

the intestinal

antagonists

agents

the

also plays an integral

etiologic

and

that underlie

that

genes

inflammatory

Recent

Moreover,

to dis-

elucidate

of the acute and chronic

cytokines within

HLA

to disease

the specific

produced

as yet undefined

should

system

in

unknown.

these diseases

Intestinal

those

remain

flammatory

mechanisms

tion.

by which

inflammation

DQ2

to be neces-

also contribute

studies

immune

although

nomenclature the specific

other

contribute

role in the pathogenesis (IBD),

the development

regarding

micro-

(new

factors

Future

factors

The intestinal

and or in

22 Although

susceptibility,

1279

(DQB1*0201

by those genes appears

mechanisms

ronmental

with

development.

encoded

TRACT

DRI 7 haplotypes

DR5

haplotypes.

sary for disease

cosal

M

heterozygous

molecule

specific

to deliv-

to selectively patch

on

DRl l)/DR7

array of different

particulates

and strategies

delivery

vac-

including

approaches

a broad

tram

of genes

m . cis on HLA

ease susceptibility.

as the use of biodegradable

incorporate

of a pair

mucosal

to develop

Other

presence

DQA1*05011)

genes or environmental

virus, as well as recombinant

vectors.

such

which

have

the

immunogens.

array of approaches

the use of live attenuated viral and bacterial

mucosa

in the mucosa enteric

attempts

a broad

of oral toler-

in the intestinal

as effective

current

ery systems

phenomenon

persist

physi-

system (e.g., the role of patterns, unique fea-

that replicate

recognized

Therefore,

of the normal

OF THE INTESTINAL

delivery

systems.

of new

mucosal

vaccines

Studies of immunogenet-

strength of its genes. Thus, as

ics are leading to new insights into the molecular basis of diseases such as celiac disease, IBD, and hepatic disorders. Studies of cytokines and the role they play in acute and chronic inflammation are leading to new

shown by Tosi et a1.21 over 90% of patients with celiac disease carry an HLA DQ2 molecule (this locus was formerly termed DC). 21 More recently, it was shown that inheritance of this DQ molecule is determined by

approaches for the treatment of intestinal inflammatory diseases. The field of intestinal immunology is now well on its way through the turbulent “teenage” years. It is true maturation over the next several de-

testinal disease based on the remarkable association with HLA class II D region

1280 MARTIN F. KAGNOFF

cades will test and witness the evolution and validity of concepts formed over the past 50 years. Certainly, tremendous growth and change will come with the increasing application of molecular tools, and the enthusiasm and the fresh insights brought by the next generation of young investigators, as they begin to further unravel the intricacies of the intestinal immune system and its role in health and disease.

References 1. Chodirker WB, Tomasi TB. Gamma-globulins. Quantitative relationships in human serum and nonvascular fluids. Science 1963; 142:1080-1081. 2. Newcomb RW, Normansell D, Stanworth DR. A structural study of exocrine IgA globulin. J lmmunol 1968; 10 1:905-g 14. 3. Halpern MS, Koshland MR. Novel subunit in secretory IgA. Nature 1970;228: 1276- 1278. 4. Mestecky J, Zikan J, Butler WT. lmmunoglobulin M and secretory immunoglobulin A: presence of a common polypeptide chain different from light chains. Science 197 1; 17 1: 1163- 1165. 5. Vaerman JP, Heremans JF. Subclasses of human immunoglobulin A based on differences in the alpha polypeptide chains. Science 1966; 153:647-649. 6. MacDermott RP, Nahm MH. Expression of human immunoglobulin G subclasses in inflammatory bowel disease. Gastroenterology 1987;93:1127-1129. 7. Kaetzel CS, Robinson JK, Chintalacharuvu KR, Vaerman JP, Lamm ME. The polymeric immunoglobulin receptor (secretory component) mediates transport of immune complexes across epithelial cells: A local defense function for IgA. Proc Natl Acad Sci USA 1992;23:1-12. 8. Crabbe PA, Heremans JF. Selective IgA deficiency with steatorrhea. A new syndrome. Am J Med 1967;42:319-326. 9. Craig SW, Cebra JJ. Peyer’s patches. An enriched source of precursors for IgA-producing immunocytes in the rabbit. J Exp Med 197 1; 134: 188-200. 10. Bockman DE, Cooper MD. Pinocytosis by epithelium associated with lymphoid follicles in the bursa of Fabricius, appendix, and Peyer’s patches. An electron microscopic study. Am J Anat 1973; 136:455-478. 11. Owen RL, Jones AL. Epithelial cell specialization within human Peyer’s patches: an ultrastructural study of intestinal lymphoid follicles. Gastroenterology 1974;66: 189-203.

GASTROENTEROLOGY Vol. 105. No. 5

12. Groh V, Porcelli S, Fabbi M, Lanier LL, Picker U, Anderson T, Warnke RA, Bhan AK, Strominger JL, Brenner MB. Human lymphocytes bearing T cell receptor gamma/delta are phenotypically diverse and evenly distributed throughout the lymphoid system. J Exp Med 1989; 169: 1277- 1294. 13. Rodgers VD, Fassett R, Kagnoff MF. Abnormalities in intestinal mucosal T cells in homosexual populations including those with the lymphadenopathy syndrome and AIDS. Gastroenterology 1986;90:552-558. 14. Eckmann L, Kagnoff MF, Fierer J. Epithelial cells secrete the chemokine interleukin-8 in response to bacterial entry. Infect lmmun (in press). 15. Eckmann L, Jung HC, Schuerer-Maly CC, Panja A, MorzyckaWroblewska E, Kagnoff MF. Differential cytokine expression by human intestinal epithelial cell lines: regulated expression of interleukin 8. Gastroenterology 1993 (in press). 16. Kim P-H, Kagnoff MF. Transforming growth factor pl increases IgA isotype switching at the clonal level. J lmmunol 1990; 145: 3773-3778. 17. Kagnoff MF. Immunological unresponsiveness after enteric antigen administration. In: Strober W, Hanson LA, Sell KW, eds. Recent advances in mucosal immunity. New York: Raven, 1982: 95-111. 18. Chase MW. Inhibition of experimental drug allergy by prior feeding of the sensitizing agent. Proc Sot Exp Biol Med 1946;61: 257-259. 19. Miller A, Lider 0, Roberts AB, Sporn MB, Weiner HL. SuppressorT cells generated by oral tolerization to myelin basic protein suppress both in vitro and in vivo immune responses by the release of transforming growth factor beta after antigen-specific triggering. Proc Natl Acad Sci USA 1992;89:42 l-425. 20. Falchuk JM, Rogentine GN, Strober W. Predominance of histocompatibility antigen HL-A8 in patients with gluten-sensitive enteropathy. J Clin Invest 1972;5 1: 1602- 1605. 21. Tosi R, Vismara D, Tanigaki N. Evidence that coeliac disease is primarily associated with a DC locus allelic specificity. Clin Immunol lmmunopathol 1983;28:395-404. 22. Kagnoff MF, Harwood JH, Bugawan TL, Erlich HA. Structural analysis of the HLA-DR, -DQ, and -DP alleles on the celiac disease-associated HLA-DR3 (DRwl7) haplotype. Proc Natl Acad Sci USA 1989;86:6274-6278. Address requests for reprints to: Martln F. Kagnoff, M.D., Department of Medicine, 062313, Unlversity of California, San Diego, 9500 Gllman Drive, La Jolla, California 92093-0623.