Grass Pollen Antigens

Worldwide, grass pollen sensitivity is the most common cause of allergic disease. This is because of the wide distribution of wind-pollinated grasses. Important grass species involved in allergic reactions are Lolium perenne (ryegrass), Phleum pratense (timothy), Poa pratensis (June grass, Kentucky bluegrass), Festuca pratensis (meadow fescue), Dactylis glomerata (cocksfoot, orchard grass), Agrotis tenuis (redtop), Anthoxanthum odoratum (sweet vernal), Sorghum halepense (Johnson grass), and Cynodon dactylon (Bermuda grass). The last two are subtropical grasses, whereas the others are temperate grasses.

Grass pollens differ from ragweed pollen in their allergenic and antigenic properties, and offer additional immunologic perspectives because of their extensive cross-reactivity. In addition, in contrast to ragweed, grasses typically release their pollen grains in the afternoon. Among the grasses, ryegrass and timothy have been most extensively studied (12,67,68).

Examination of a number of allergenic grass pollen extracts by immunochemical methods has disclosed between 20 and 40 different antigens. Further analysis of these components has shown that some are more able than others to bind IgE from the serum of allergic patients or to produce positive skin test results. Some of these are major allergens in that they produce skin test reactivity or demonstrate IgE binding in more than 50% of grass-sensitive patients.

Several grass pollen allergens have been isolated and categorized into eight groups based on chemical and immunologic characteristics. They are as follows: I, II, III, IV, IX (V), X, XI, and the profilins. Within each group, several individual allergens have been identified that are similar immunochemically and are extensively cross-reactive.

The group I allergens are located in the outer wall and cytoplasm of the pollen grains, as well as around the starch granules ( 69). These small (3 pm diameter) granules are readily released on contact with water. Two representative members of the group I grass allergens are Lol p 1 (ryegrass) and Phl p 1 (timothy). Despite the fact that both of these allergens have been sequenced and cloned, their biochemical identity is not known with certainty. Studies of group I allergens in maize isolated with antibody against Lol p 1 suggest that the group I antigens may act as "cell wall-loosening agents" (70). High cross-reactivity between the group I allergens from different grass species has been observed, including similarities in IgE RAST inhibition, crossed immunoelectrophoresis (CIE), and monoclonal antibody mapping (71,72 and 73). Indeed, amino acid sequences document homologies among these group I members (74). Other studied group I members include Poa p 1 (Kentucky bluegrass), Cyn d 1 (Bermuda), Dac g 1 (orchard), and Sor h 1 (Johnson). The group I allergens are of major importance in that by skin testing and histamine release, 90% to 95% of grass pollen-allergic patients react on testing ( 75). Groups II and III show significant but lesser degrees of reaction, varying between 60% and 70% of patients (67).

There is a relative paucity of data regarding the group II, III, and IV grass allergens. Group II allergens include Lolp 2, a ryegrass allergen that has been cloned and expressed as a recombinant molecule in a bacterial vector (76). Forty-five percent of ryegrass-allergic patients react to this allergen. Profilin, a compound involved in actin polymerization, has been described as a component of several tree pollens (77). It is allergenic and also has been found to be a minor allergen in the grass allergen group II family, in addition to several weed species.

Lol p 3 and Dac g 3 have both been sequenced and cloned. Despite 84% identity, the predicted secondary structures suggest they may not be cross-reactive ( 78). Group IV allergen from timothy grass has been characterized and found to have a significant cross-reactivity with Amb a 1 (79). Only about 20% of grass pollen-sensitive patients appear to be skin test reactive to these allergens.

Groups V and IX (now grouped together as IX) are a heterogeneous group of proteins. Group IX allergens from Kentucky bluegrass, ryegrass, and timothy grass all have been sequenced and cloned. Analysis of the cloned Kentucky bluegrass allergen, Poa p 9, has suggested the existence of a family of related genes. When compared with the ryegrass allergen, Lol p 9, a 44% homology is seen (80). No other members of group IX show this level of homology. Among the group V allergens, the most work has been done with the timothy grass allergens Phl p 5a and Phl p 5b. These allergens have been cloned and identified as novel pollen RNAses, which may play a role in host-pathogen interactions in the mature plant (81). Other group V allergens have been isolated from a number of temperate grasses, including Dactylis glomerata (orchard grass). The Dac g 5b allergen also has been cloned and coded for a fusion protein that was recognized by IgE antibodies in six of eight samples of atopic sera tested. This suggests that Dac g 5b may be a major allergen, but it has not been completely characterized (82).

The most recent major grass pollen to be identified, Lolp 11, appears to be a member of a novel allergen family (83). No sequence homology with known grass pollen allergens was found, but it does have 32% homology with soybean trypsin inhibitor ( 83). This allergen reacted with IgE from over 65% of grass-pollen positive sera tested. Lol p 11 appears to share some sequences with allergens from olive pollen, as well as tomato pollen. The cDNA of Cyn d 7 also has been cloned recently and has two calcium binding sites. Depletion of calcium causes a loss of IgE reactivity ( 84).

The cDNA cloning of multiple grass allergens has some potential diagnostic applications. A strategy to take advantage of the extensive cross-reactivity between species using recombinant allergens has been studied. A mixture of Phl p 1, Phl p 2, Phl p 5, and Bet v 2 (birch profilin) accounted for 59% of grass-specific IgE ( 85). A study of purified Lol p 1 and Lol p 5 versus recombinant Phl p 1 and Phl p 5 was performed on RAST-positive patients. The Lol p extracts reacted with 80% of the IgE, whereas the recombinant Phl p reacted with 57% of the IgE (86).

One of the most innovative applications of DNA technology has been the development of rye grass plants with downregulation of the Lol p 5 gene. This transgenic ryegrass pollen maintained its fertility, but had a significant decrease in its IgE binding capacity compared with normal pollen. This creates the possibility of genetic engineering of less allergenic grasses (8.7).

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