F ig. 3
www.alergiainfantillafe.org
It’s been calculated that patients sensitized can have symptoms with a concentration from 10 to 50 grains of grass pollen, depending on the particular severity of the allergy of each patient, the concomitance with other allergies to plants pollinating at the same time, weather conditions or level of exposure (for example higher levels when cutting the lawn). The recognized allergens are usually the most numerous or the most sensitizing. A major allergen is defined because of its higher prevalence or the increased IgE percentage against it in a determined population. The standarization methods are usually directed to their detection, so that polisensitized patients can suffer a misdiagnosis or a therapeutic failure for that reason. Major allergens are usually more specific while minor usually include several panallergens responsible for most phenomena of cross reacting among pollens and vegetable foods (3).
Table II resumes what actually known about grass pollen allergens, where Lolium and Cynodon species are the most studied. There are more than 12 allergen families identified in grasses. Two majoritary allergens (Groups 1 and 5) and two panallergens responsible of cross reacting (Groups 7 and 12) are the best known (1) although there is a variety of proteins of different sizes, structures and physicochemical properties existing as multiple isoforms (3).
TABLE II- GRASS POLLEN ALLERGENS
Pollen Allergen Biologic function Mw(kDa) Sequency IgE prevalence(%)
C dactylon Cyn d 1, 7, 12
7 Calcium binding 12 C 10
12 Profilin 14 C 20
D glomerata Dac g 1, 2, 3, 5
H lanatus Hol l 1 34 C 67
Hol l 5 30 C 64
H vulgare Hor v 5 30 C +
L perenne Lol p 1, 2, 3, 5, 10, 11
10 Citochrome C 12 P -
11 Trypsine inhibitor 16 C 65
O sativa Ory s 1 35 C +
Ph aquatica Pha a 1 34 C 77
Pha a 5 31-33 C 42
Phl pratense Phl p 1, 2, 4, 5, 6, 7, 12, 13
5 Ribonuclease 32-38 C 80
6 P particle associated 13 C 75
7 Calcium binding 6-8 C 10
12 Profilin 14 C 20
13 Poligalacturonase 55-60 C 50
P pratensis Poa p 1, 5, 10
10 Citochrome C 12 P -
S halepense Sor h 1 35 C +
Z mays Zea m 1 Expansine 17-33 C +
Zea m 12 Profilin 14 C +
Grass pollen allergens 1 and 5
They are recognized by about 80-90% of people sensitized to grass pollen having specific IgE against them. The recognizition of only one grass species seems to be sufficient for in vitro diagnosis of grass pollen allergy. With purified Lol p 1 and Lol p 5, many more than 50% of grass positive sera are detected. Around 80% of the IgE response to grass pollen is directed to these major allergens as it has been shown in inhibition assays (4).
Grass group 1 pollen allergens are 27-35 kDa glycoproteins, functionally belonging to the group of - expansins (5). The different species show a high homology (Lol p 1 is very similar to Phl p 1 as an example), not so evident in other species such as Cynodon, where Cyn d 1 is only 50% similar to Lol p 1. This fact and the apparent absence of Cyn d 5, show specific sensitization patterns for Cynodon dactylon. - expansins are defined by their characteristic sequence and by their unique rheological effects on plant cell walls, which include the rapid induction of cell wall extension and the enhancement of wall stress relaxation. These proteins (divided into and families) are believed to be the key catalysts of cell wall loosening, necessary for plant cell growth, cell separation (abscission) and other related processes (studies done over Zea m1 allergen). - expansins are more common than , presenting a high concentration, solubility and a weak interaction with cellular walls, what lets them to be more allergenic because of their high biodisponibility (6).
Grass group 5 allergens seem to be exclusive of the Poaceae subfamily. Their molecular weight is 28-40 kDa, not having any glycosilation in its molecule. They have RNAse activity and are suggested to participate in defense reactions of the plant (7). The more relevant characteristic of this group of allergens is the high presence of different allergenic isoforms. Data obtained from studies in Phleum show there are two separate groups of isoallergens Phl p 5a (31 kDa) and Phl p 5b (28 kDa). Although there not seems to be any difference among isoforms when binding IgE, there are studies where different T cell recognition patterns have been found, what can be interesting to develop different therapeutic strategies based on recombinant allergens (8). Their concentration in the pollen grain is high and they are weakly associated to starch particles in aerosols what enhances their allergenicity.
Grass pollen panallergens
Two big groups can be distinguished:
Group 7: calcium-binding proteins.
Group 12: profilins
Calcium-binding proteins (group 7)
The group is integrated by 32 subfamilies, having from 2 to 8 calcium-binding sites, and present in different vegetal species. Their capacity of binding calcium (Ca++) makes them different whether it is in the molecule or not. Phl p 7, proved to be a primary sensitizing allergen from inhibition studies (9), and Cyn d 7 are the two better known allergens, although only 10 to 20% of the grass pollen allergic patients are sensitized to them. They seem to be clinically relevant, responsible for high cross reacting with other pollen antigens. They are also considered to be homologous with some human proteins, being able to cause autoimmune diseases with IgE production such as severe atopic dermatitis (10).
Profilins (Group 12)
They are present in many biological systems, having a relevant role in actin-binding regulation, although not yet completely understood (11). They have been proved to mediate in the grass pollen tube growth, but only about 20% of patients are sensitized to them as an allergen. They are considered as extremely ubiquitous, present in different tissues such as storage ones, and possibly responsible of cross-reacting between pollen and vegetable foods as Rosaceae or legumes (12). The photo shows the molecular model of the Phleum profilin (Phl p 11), courtesy of Bial-Aristegui R+D department (Bilbao, Spain).
Fig 4. 
Share with your friends: |
