Timothy grass

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Code: g6
Latin name: Phleum pratense
Source material: Pollen
Family: Poaceae (Gramineae)
Sub family: Pooideae
Tribe: Agrostideae
Common names: Timothy, Herd's Grass, Cat's Tail
Synonyms: P. nodosum, P. parnassicum
 
The two commonly recognised varieties are
  • P. Pratense var. pratense
  • P. Pratense var. nodosum
A grass species producing pollen, which often induces hayfever, asthma and conjunctivitis in sensitised individuals.

Allergen Exposure

Geographical distribution
Timothy is native to Europe, North Africa and Northern Asia, and has been introduced and widely cultivated as a hay and pasture grass in North and South America, South Africa and Australia. It is one of the world's most common grasses and one of the most common sources of animal fodder. It grows best in cooler, humid climates.
 
Timothy grows to 1m tall. It is a clump-forming, rather short-lived perennial with characteristic long, cylindrical, spike-like, somewhat purplish to silvery flower heads on wiry stems. The stem often bends at the lower nodes. Sheaths are open with edges inrolled. Leaf blades are 4 to 10 mm wide, flat and tapering. The leaf blades on the mid- to upper-flowering stem are much shorter (3 to 10cm long) than the sheaths.
 
Timothy flowers from early summer to midsummer. Just before or during flowering is the usual time for hay cutting, a circumstance that may reduce pollen exposure. The flowers are hermaphrodite (have both male and female organs).
 
The plant is wind-pollinated. The seeds produced are among the smallest seeds of the common pasture species. The plant reproduces mainly from seed, but vegetative reproduction also occurs.
 
Phleum pratense is sometimes confused with Agrostis alba (a Redtop or Bentgrass) but can be distinguished by its white and more opaque ligule with a notch at either side and without hairs on the back. The presence of cilia on the shoulder and the less conspicuous ridging of the upper surface of the blade are also diagnostic.
 
Environment
Timothy is widespread in fields and meadows, and on roadsides. It is sown in pastures for forage, and is very common in hay. This most extensively cultivated of meadow grasses escapes freely and becomes established in natural meadows and waste places.
 
Allergens
The protein content of Timothy grass pollen varies from season to season, which may impact on the total allergen load that sensitised individuals are exposed to (1). In addition, pollen freshly collected on rural meadows and near high-traffic roads demonstrates a striking difference according to origin, with higher allergen release rates from rural meadow pollen grains (2).
 
P. pratense contains at least 28 antigens, of which 15 have been shown to bind to IgE(3). A number of major allergens have been detected (4). 
  • Phl p 1, a major Group 1 allergen (5-6) 
     
  • Phl p 4 (7-8) 
     
  • Phl p 5, a major Group 5 allergen (9-10)
      
  • Phl p 6, a 11-12 kDa protein (11-12) 
     
  • Phl p7, a calcium-binding protein (13-14) 
     
  • Phl p 12, a 14.2 kDa protein, a profilin (an earlier name is Phl p 11) (15-17) 
     
  • Phl p 13 (8, 18) 
Two Group 1 allergens, isoforms of Phl p 1, have been detected, and comprise 37 and 35 kDa components (19).
 
Phl p 5 is the dominant allergen of P. pratense, and consists of two isoforms having apparent molecular weights of 38 kDa (Phl p 5a) and 32 kDa (Phl p 5b). Each isoform is split into at least four isoallergens(20). Furthermore, Phl p 5-specific T cells are highly heterogeneous. Individual T-cell clones, and individual patients, differentially recognise isoallergens (21).
 
More than 70% of the patients allergic to Timothy grass pollen exhibit IgE-reactivity against the high molecular mass fraction between 50 and 60 kDa of Timothy grass pollen extracts. One allergen from this fraction is Phl p 4, and another Phl p 13. Using sera of 306 subjects allergic to grass pollens, specific IgE-binding to both Phl p 4 and Phl p 13 was demonstrated for more than 50% of subjects. There were clear differences in the immunological properties of Phl p 4 and Phl p 13 (7).
 
In an Austrian study, Phl p 6 reacted with serum IgE from 75% of 171 grass pollen-allergic patients (11).

Potential Cross-Reactivity

An extensive cross-reactivity among the different individual species of the genus could be expected, as well as to a certain degree among members of the family Poaceae, in particular grasses belonging to the subfamily Pooideae (Rye grass (g5), Canary grass (g71), Meadow grass (g8), Timothy (g6), Cocksfoot (g3), Meadow Fescue (g4), Velvet grass (g13), Redtop (g9), Meadow Foxtail (g16), Wild Rye grass (g70)) (22, 23). 
  
This grass contains Group 1 allergens, to which more than 95% of patients allergic to grass pollen possess IgE antibodies. These are highly cross-reactive glycoproteins exclusively expressed in the pollen of many grasses (24, 25, 26).
 
Group 1 allergens are highly homologous, but not all of the antigenic epitopes are crossreactive (27). For example, Group 1 allergens from eight different clinically important grass pollens of the Pooideae (Rye grass, Canary grass, Meadow grass, Cocksfoot and Timothy), Chloridoideae (Bermuda grass) and Panicoideae (Johnson grass, Maize) were isolated, and IgE binding to an allergic human serum pool was conducted to determine the degree of antigenic and IgE-binding similarities. The highest IgE-binding similarity was observed between Cocksfoot and Rye grass (53%) and between Rye grass and Canary grass (43%). No IgE-binding similarity was observed between Maize and other grasses. The highest antigenic similarity was also observed between Rye grass and Cocksfoot grass (76%), and the lowest similarity between Maize (23%) and Bermuda (10%)(28). 
 
In another study highly homologous Group 1 allergens have been demonstrated between Pha a 1 from Canary grass, Lol p 1 from Rye grass pollen (a deduced amino acid sequence identity of 88.8%), Hol l 1 from Velvet grass pollen (88.1%), and  Phl p 1 from Timothy grass pollen (86.6%) (29). The major Timothy grass pollen allergen Phl p 1 also cross-reacts with most grass-, Corn- and monocot-derived Group 1 allergens (5). Monoclonal antibodies of Cyn d 1 (Bermuda grass) recognised cross-reactive epitopes on proteins from eight other grasses including Rye grass, Timothy grass, Meadow grass and Johnson grass (30).
 
T-cell lines specific for Phl p 1 (the Group 1 allergen of Timothy grass) from the sera of 9 patients allergic to grass pollen displayed IgE binding with grass pollen extracts from Cocksfoot, Meadow grass, Rye, and Cultivated Rye, and selected amino acid sequence-derived peptides. Cross-reactivity studies revealed cross-reacting and non-cross-reacting T-cell epitopes (31).
 
Timothy grass also contains a Group 4 allergen. Group 4 grass pollen allergens are glycoproteins with a molecular weight of 50 to 60 kDa, which are present in many grass species. Almost 75% of patients allergic to grass pollen display IgE reactivity to Group 4 allergens, which hence can be regarded as major grass pollen allergens (7). Phl p 4 represents a trypsin-resistant major Timothy grass pollen allergen with immunologic similarities to the major ragweed allergen Amb a 1 and therefore must be considered an important cross-reactive component in grass pollen and weed pollen allergy (7). Inhibition studies of IgE antibody binding to Dac g 4 (Dactylis glomerata - Cocksfoot grass) and to other pollen extracts confirmed the presence of cross-reactive allergens in Secale cereale (Cultivated Rye), Lolium perenne (Rye grass), Festuca elatior (Meadow Fescue), Holcus lanatus (Velvet grass), Bromus arvensis (Field Brome), Poa pratense (Meadow grass), Hordeum sativum (Barley), and Phleum pratense (Timothy grass) (32). 
 
Oilseed Rape pollen proteins were capable of quenching IgE binding to Timothy grass pollen proteins of >/=60 kDa, suggesting that grass pollen Group 4 allergens cross-react with the 27 to 69 kDa cluster in Oilseed Rape pollen (33).
 
Timothy grass pollen also contains a Group 5 allergen. Almost 90% of grass pollen-allergic patients are sensitised against Group 5 grass pollen allergens.
 
Group 5 allergens have been detected in Timothy, Rye grass, Meadow grass and Cocksfoot extracts. The major components in these fractions were found to be 25-28 kDa proteins, and IgE binding to these components was confirmed using a pool of grass-allergic sera (10).

A monoclonal human IgE antibody has been shown to cross-react with Group 5A isoallergens from several grass and Corn species (9). Polymorphic forms of Pha a 5 from Canary grass have been shown to share significant sequence identity with other Group 5 allergens from Rye grass, Timothy and Meadow grass pollens (29).
Group 5 allergens of Timothy grass (Phl p 5) bear T cell epitopes cross-reacting with Group 1 allergens of Rye grass (Lol p 1) (34). 
 
The variability of cross-reactivity of IgE antibodies to Group 1 and 5 allergens in 8 grass pollen species was examined. Cross-reactivity of IgE antibodies against Lol p I or Lol p V (from Rye grass) to Dactylis glomerata (Cocksfoot), Festuca rubra (Red Fescue), Phleum pratense (Timothy), Anthoxanthum odoratum (Sweet Vernal grass), Secale cereale (Cultivated Rye), Zea mays (Maize), and Phragmites communis (Dust Mite) was investigated by means of RAST-inhibition. Within a group of sera the degree of cross-reactivity was demonstrated to be highly variable. Individual sera were not always equally cross-reactive to all pollen species. A high degree of cross-reactivity for Group 1 allergens did not necessarily imply the same for Group 5. Group 1 and Group 5 representatives were found to be present in all 8 species (35).
 
Sequence comparisons showed that the Hor v 9 cDNA clones (Barley pollen) were also homologous to Group 5 allergens of Timothy grass pollen and Canary grass (Phalaris aquatica) pollen, and the Group 9 allergen of Ryegrass (Lolium perenne) pollen (36). 
 
Timothy grass contains a Group 13 allergen. Group 13 comprises major allergens in the grasses, allergens which have molecular masses of 50 to 60 kDa, are detectable in many common grasses, and show IgE cross-reactivity among themselves (18).
 
Timothy grass contains a calcium-binding protein, to which Group 13 allergens share similarities that may result in cross-reactivity (13).
 
Timothy grass also contains a profilin, which is in part responsible for the T-cell-mediated immunological response in patients allergic to Phl p. The response is very specific, since Phl p profiling-specific T-cell lines did not show cross-reactivity with a highly homologous profilin from Wall Pellitory (15). A study reported that binding to Camomile was inhibited in variable degrees by extracts from Celery roots, Anise seeds and pollen from Mugwort, Birch and Timothy grass, but the allergen was shown not to be a profilin (37).
 
Bahia and Timothy grass have in some studies been shown not to be cross-reactive (38, 39).  
 
Timothy grass seem to share allergens with Kiwi extract (40).
 
Mugwort, Ragweed, and Timothy grass pollen also share IgE binding  epitopes with glycoprotein Latex allergens. The presence of common epitopes might in part explain clinical symptoms in pollen-allergic patients on contact with Latex. Any previously known panallergen was not detected (41).
 
Monoclonal antibodies specific for the major Mugwort pollen allergen Art v 1, approximately 60 kDa in size, were shown to result in a reduction of IgE binding to components of a similar molecular weight present in Timothy grass. The allergen was reported to be distinct from Bet v 1 and profilin and hence may represent a novel cross-reactive allergen in Oral Allergy Syndrome (42).
 
Immunological identity has been demonstrated between recombinant Dac g 2 (from Cocksfoot grass) and Lol p 1 and Lol p 2 (both from Rye grass), and similar cross-identity was observed with pollen extracts from 3 other grass species: Festuca rubra (Red Fescue), Phleum pratense (Timothy grass) and Anthoxanthum odoratum (Sweet Vernal grass) (43).

Clinical Experience

IgE mediated reactions
Allergy to Timothy grass pollen has been reported widely. Timothy grass is one of the most important causes of allergic rhinitis, asthma and allergic conjunctivitis during summer in cool temperate climates (44, 39, 45).  
 
A European Community respiratory health survey reported that adults who had lived on farms as children were less frequently sensitised to Timothy grass, and were at lower risk of having nasal symptoms in the presence of pollen in general(46).
 
Timothy grass is a very prevalent aeroallergen in the Mediterranean countries, including Spain (47, 48, 49). In Salamanca, in a study in a Birch- and Ragweed-free area, 97.9% of patients allergic to pollens were sensitised to Timothy and Rye grass(50). The strongest associations between bronchial hyper-reactivity and specific IgE responses were seen with Timothy grass (51).
 
Timothy grass has also been shown by specific IgE determination to be an important allergen in the Netherlands (52).
 
In Sweden, in specific IgE tests on 7099 adult patients with asthma and/or rhinitis, 44% of patients were positive, decreasing from 61% in patients 14-20 years old to 18% in patients 61-70 years old. Timothy, Cat and Birch were the most prevalent allergens. Of these patients, 65% were sensitised against several allergens and 35% had a mono-allergy, most frequently to Timothy grass (70%) (53).
 
In Norway, in 770 patients with seasonal and perennial nasal symptoms, pollens from Timothy, Meadow Foxtail, Meadow grass and Meadow Fescue were found to be very important causative factors (54).
 
Timothy grass pollen was reported as the most prevalent sensitising allergen in Estonia, with 4.8% of 1519 schoolchildren aged 10-12 years sensitised to this grass (55).
 
Similarly in Turkey, in a study of 1149 patients with asthma from 5 major cities, the most common grass allergen to which patients were sensitised was Timothy grass, the prevalence varying from 4 to 19% (56).
 
In Japan, Timothy grass has been shown by specific IgE and other investigations to be a very prevalent allergen(57,58). In 107 patients with nasal allergies in Sapporo, RAST was positive for Timothy in 22.4%.
 
Timothy pollen is also an important aeroallergen in Thailand. In 100 patients with allergic rhinitis, sera-specific IgE to Timothy grass was raised in 16% (59).

Compiled by Dr Harris Steinman, harris@zingsolutions.com

References

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As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.