major histocompatibility complex

mhc

and mouse urine

odor
http://www.pnas.org/cgi/content/full/94/6/2210



Variation in the genes of the major histocompatibility complex (MHC) contributes to unique individual odors

(odortypes) in mice, as demonstrated by the ability of trained mice in a Y-maze olfactometer to discriminate nearly

identical inbred mice that differ genetically only at the MHC (MHC congenic mice), while they cannot distinguish

genetically identical inbred mice. Similar distinctions are possible with urine, a substance that is involved in

many facets of mouse chemical communication. This paper reports results supporting the hypothesis that the

MHC-determined urinary odor is composed of a mixture of volatile carboxylic acids occurring in relative

concentrations that are characteristic of the odortype. Y-maze behavioral testing of urine fractions from anion

exchange chromatography indicates that volatile acids are necessary and sufficient to convey MHC odortype

information. Diethyl ether extracts, which are expected to contain the more volatile, less polar organic acids, were

also discriminable in the Y-maze olfactometer. Ether extracts of 12 different urine samples from each of two panels

of MHC congenic mice were analyzed by gas chromatography. No compounds unique to urine of either genotype were

detected, but compounds did appear to occur in characteristic ratios in most of the samples of each type.

Nonparametric statistical analysis of the gas chromatographic data showed that eight of the peaks occurred in

significantly different relative concentrations in the congenic samples. One of the peaks was shown to represent

phenylacetic acid, which has implications for the mechanism of the MHC specification of odortype.

Evidence for

MHC-correlated perfume preferences in

humans
http://beheco.oxfordjournals.org/cgi/c

ontent/abstract/12/2/140

Fragrances have been used since at least 5000 years ago and all traditional

scents are found in modern perfumes. Although perfumes are obviously involved in sexual communication, the

significance of great individual differences in preference for fragrances is an evolutionary puzzle. The major

histocompatibility complex (MHC) is a highly polymorphic and conserved set of genes that plays an important role in

immune function in vertebrates. Both mice and humans have been shown to prefer the body odor of potential partners

that have a dissimilar MHC genotype, which would result in heterozygous offspring. We tested whether individual

preferences for perfume ingredients correlate with a person's MHC genotype. The human MHC is called HLA (human

leukocyte antigen). A total of 137 male and female students who had been typed for their MHC (HLA-A, -B, -DR) scored

36 scents in a first test for use on self ("Would you like to smell like that yourself?") and a subset of 18 scents

2 years later either for use on self or for a potential partner ("Would you like your partner to smell like that?").

An overall analysis showed a significant correlation between the MHC and the scorings of the scents "for self" in

both tests. In a detailed analysis we found a significant interaction of the two most common HLAs with the rating of

the 36 scents in the first study as well as with the 18 scents in the second study when evaluated for self. This

result suggests that persons who share, for example, HLA-A2, have a similar preference for any of the perfume

ingredients. The significant repeatability of these preferences in the two tests showed that the volunteers that had

either HLA-A1 or HLA-A2 were significantly consistent in their preferences for the perfume ingredients offered.

Hardly any significant correlation between MHC genotype and ratings of the scents "for partner" were found. This

agrees with the hypothesis that perfumes are selected "for self" to amplify in some way body odors that reveal a

person's immunogenetics.

MHC genes, body odours, and odour

preferences
http://ndt.oxfordjournals.org/cgi/conte

nt/full/15/9/1269

Increasing evidence indicates that the highly polymorphic genes of the major

histocompatibility complex (MHC) influence odour and mating preferences in house mice and humans [1]. MHC genes

encode cell-surface glycoproteins (class I and II molecules) that bind short peptides and present them to T

lymphocytes. Through this mechanism, MHC genes control the immunological self/non-self discrimination, and

subsequently, tissue rejection and immune recognition of infectious diseases. Thus, it is suspected that the

extraordinary polymorphism of MHC loci is maintained by balancing selection from infectious diseases, though direct

evidence for this hypothesis is lacking [2]. Surprisingly, the best evidence indicates that MHC polymorphisms are

driven by sexual selection. Studies in house mice indicate that both males and females prefer MHC-dissimilar mates

[3–5], who they apparently recognize by odour cues [6]. Studies in humans have also found MHC-associated odour [7]

and mating preferences [8]. Such disassortative mating preference could explain the diversity of MHC genes, though

several questions remain unanswered about how the MHC influences odour production and why MHC-dependent mating

preferences evolved.

There is much evidence that MHC genes influence individual odour in laboratory mice and

rats (reviewed in [20]), and several hypotheses have been proposed to explain how. First, since MHC molecules occur

in the urine and sweat, they may provide the odourants [21]. This is unlikely since MHC molecules are large,

involatile proteins, and furthermore, denaturation of proteins in urine does not destroy the distinguishability of

MHC-mediated odours by mice [22]. Second, MHC molecules bind to allele-specific subsets of peptides, and their

volatile metabolites, such as carboxylic acids, may provide the odourants. Class I MHC molecules bind peptides that

are hydrophilic, highly evolutionarily conserved, universally expressed and derived from hydrophobic proteins,

whereas Class II- bound peptides are more conserved than their source proteins but less conserved than class I-bound

peptides [23]. Singer et al. [24] found that the relative concentrations of volatile carboxylic acids were

characteristic of the urinary odour of different MHC-congenic inbred mouse strains. Third, MHC genes may alter odour

by shaping specific populations of microbial flora, although the evidence for this idea is inconsistent [25–27].

Fourth, MHC molecules may change their conformation to bind volatiles, instead of peptides, and carry them to scent

glands [28]. Finally, when taken together, the evidence suggests that MHC-bound peptides are metabolized and made

volatile by microbes [20]).

Odortypes: Their origin and

composition
[url="http://www.pnas.org/cgi/content/full/96/4/1522"]http://www.pnas.org/cgi/content/full/96/4/1522[/u

rl]

Odors that distinguish one individual from another member of the species and are determined by polymorphic

genes are called odortypes. Odortypes and their considerable societal significance have been studied intimately only

in mice and mainly with respect to the genes of the major histocompatibility complex. Further understanding and the

matter of human relevance have been hampered by the apparent restriction of odortype expression to urine. The

present finding that odorants comprising prerenal odortypes are already present in blood, albeit in masked form,

affords the basis of a comprehensive view of odortypes. Accordingly, major histocompatibility complex and other

polymorphic genes of antiquity are seen inter alia as agents of normal variation, which entails quantitative

variation in output of odorant metabolites. Relatively few such normal variations should suffice for a vast range of

compound odors whose specificity is determined by combinative assortment of the same set of individual volatile

compounds.

We recently have reported (20) evidence for distinctive patterns of volatiles according to MHC type.

In a behaviorally active dimethyl ether extract of acidified urine a series of carboxylic acids has been found that

distinguishes male mice differing only at the MHC. Behavioral tests suggest that most or all of the signal from H-2

resides in this active fraction although this fraction does not, itself, smell "mouse-like" (unpublished

observations). Although mass spectrometry indicates the presence of neutral compounds as well as the acids, these

have not yet been implicated in the chemical differences between samples of urine from congenic mice. Because these

volatile acids are abundant (approx 1 mg/ml) and strongly odorous in mouse urine, it seems probable that they play a

critical part in the olfactory discrimination of MHC-congenic mice.

Because the pattern of odorants

characterizing the MHC-determined odortype is sufficiently similar in serum and urine, it follows that the odorant

pattern is established prerenally; one proven source is the hemopoietic system (21). A likely mechanism for

odor-type specification may be that soluble MHC gene products themselves bind circulating odorants selectively,

presumably after they have lost their bound peptide, and then release them mainly during the course of renal

processing and excretion.

Although there is then a need to account for the several independent non-MHC-odortype

loci identified throughout the rest of the mouse genome (22), including both sex chromosomes (23), there is no

evidence that any of these exhibit the extensive diversity that renders the MHC unique.

Studies with H-2 mutant

mice (24, 25) and class 1 knockout mice (26) prove that MHC genes themselves, and not adjacent odorant-coding genes,

are responsible, at least in large part, for MHC odortypes.

Odortype specification and communication are by no

means alone as nonimmunological functions of the MHC, and, indeed, these may represent primordial functions of far

greater antiquity than acquired immunity (27-31). Moreover, there is reason to believe that MHC genes specify

odortypes in species other than mice, including humans, as indicated above. Such odortypes often may serve similar

purposes in different species. For example, human mating choices can be influenced by MHC genes although there is no

direct proof that body odors mediate this effect (ref. 32, but see also ref. 33). We have found that paternal MHC

type can be recognized in the scent of pregnant mice (34), and other studies implicate a similar phenomenon in

humans (35).

It is not necessary to invoke natural selection to account for the presence of MHC-regulated

odorants in body fluids; they may be natural by-products of normal MHC gene variation. Organisms as diverse as

marine invertebrates and mice and humans may have seized these serendipitously available volatile signals of

individual identity to identify appropriate mates, thereby avoiding inbreeding, or to recognize siblings, parents,

or offspring.

Finally, economy of hypothesis requires a comprehensive account of odortypes in the simplest

terms, and this is proposed here, as follows.

Odortypes are secondary, not primary, genetic traits, and so are

ubiquitous, e.g., among mammals, regardless of particular members' ability to sense them; it is a question of

olfactory ability; thus, rats distinguish the odortypes of mice with the same exquisite precision as mice themselves

(36). Visual identification among human individuals is no doubt the polymorphic anatomical parallel, quite likely

involving some of the same variable genes such as the MHC.

Lewis Thomas, in 1974 (37), founded the study of

odortypes by asking whether dogs might "sniff out our histocompatibility types for us." Whether humans can sniff out

dogs' histocompatibility types for them is a matter of interest.



[url="http://www.pnas.org/cgi/content/abstract/98/16/9249"]http://www.pnas.org/cgi/content/abstract/98/16/9249[/ur

l]

"Electronic nose" detects major histocompatibility complex-dependent prerenal and postrenal odor

components
Stefanie Montag*, Michael Frankdagger , Heiko Ulmerdagger , Dorothee WernetDagger , Wolfgang Göpeldagger

, and Hans-Georg Rammensee*,§

* Department of Immunology, Institute for Cell Biology, dagger Institute for

Physical Chemistry, and Dagger Department of Transfusion Medicine, University of Tübingen, D-72076 Tübingen,

Germany

Communicated by Edward A. Boyse, University of Arizona College of Medicine, Tucson, AZ, May 25, 2001

(received for review August 11, 2000)

Mice prefer to mate with individuals expressing different MHC genes from

their own. Volatile components presenting MHC-dependent odor types are present in urine and can be detected by mice,

as shown by extensive behavioral studies. Similar odor types are suspected to influence human behavior as well.

Although a recent report indicates that MHC expression influences the ratio of volatile compounds such as

phenylacetic acid, so far no other means than studying the behavior of mice or rats has been available to assess

odor types. Here, we report the ability of a gas sensor array (referred to as "electronic nose") to detect

MHC-dependent odor types. The electronic nose consists of an array of chemophysical detectors, in our case quartz

crystal microbalances and semiconducting metal-oxide sensors that change frequency or conductivity upon binding of

very small numbers of individual molecules present in the gas phase of odorous fluids. The pattern of changes is

characteristic for a particular smell. Our electronic nose distinguishes the urine odor types of MHC congenic mouse

strains, MHC class I mutant mice, and HLA-A2 transgenic mice. In addition, MHC-dependent odor types can be detected

in serum. The device also clearly differentiates between individual odor types of human sera from HLA homozygous

individuals; however, HLA expression seems to have only a secondary influence. Thus, odor-type research can now be

carried out with an objective and fast through-put system independent of behavioral studies.

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