g13 Velvet grass

Nature

Velvet grass is a perennial, hairy, tufted grass that grows 0.5 to 1.0 m tall (1, 2). Soft velvety leaves up to 20 cm long are the most consistent identifier for H. lanatus (2). H. lanatus has a purplish anthocyanin coloration on the panicles and veins of the sheaths (2). H. lanatus flowers from June to September in Europe and from May to October along the eastern coast of North America (1, 2). Velvet grass is wind-pollinated and produces abundant seed depending on plant age and the time at which flowers emerge (2). Common velvet grass (H. lanatus) has been reported to hybridize with creeping velvet grass (H. mollis) (1).

Taxonomy     

Allergenic molecules

The following allergens and their molecular epitopes have been characterized from H. lanatus (10).

In addition, H. lanatus pollen contains a proteinase inhibitor to bovine trypsin, comprising less than 1% of the total extractable pollen allergens, and recognized as an allergen by specific IgE antibodies in the serum of sensitized patients with pollinosis (14).

Cross-reactivity

Extensive IgE cross-reactivity has been established between velvet grass and fellow members of the subfamily Pooideae. Allergens from Group 1 and Group 5 appear to dominate the immune response to grass pollen extract, based on the prevalence of IgE antibody recognition among grass pollen-sensitized subjects (11).

Various authors have reported consistently high homology between Hol l 1 and Group 1 allergens from other Pooideae grasses. In a comparison of nucleotide sequences, there was approximately 90% sequence identity and similarity of Hol l 1 with Phl p 1 (Timothy grass, Phleum pratense), Lol p 1 (perennial ryegrass, Lolium perenne), and Pha a 1 (canary grass, Phalaris aquatica) (15); 95% homology and 93% identity with Phl p 1 in an analysis of the surface topographies of the allergens (16); and 88.1% sequence identity with Pha a 1 (17). Approximately 70% similarity has been demonstrated between Hol l 1 and Zea m 1 (maize, Zea mays) of the Poaceae subfamily Panicoideae (15), however no antigenic cross-reactivity was seen between Hol l 1 and Cyn d 1 (Bermuda grass, Cynodon dactylon) of the subfamily Chloridoideae (9).

The two isoforms rHol l 5.01 and rHol l 5.01 from H. lanatus are highly similar (range 64.6% to 83.0% sequence identity) to Group 5 allergens and their isoforms from other Pooideae homologs such as Phl p 5, Lol p 5, Pha a 5, and Poa p 5 (Kentucky bluegrass, Poa pratensis) (6). Hol l 5 harbors multiple discontinuous as well as continuous IgE-binding epitopes spread over the whole molecule, which were individually recognized by IgE antibodies from sera of 65 grass pollen allergic patients (18). Additionally, individual T cells directed against Timothy grass allergen Phl p 5.0101 cross-recognized corresponding epitopes of velvet grass, canary grass and Kentucky bluegrass (19).

More than 75% of grass pollen allergic patients produce specific IgE antibodies against Group 4 allergens (13). Inhibition studies of IgE antibody binding to Dac g 4 from orchard grass pollen (Dactylis glomerata) confirmed the presence of cross-reactive Group 4 allergens of 60 kDa in pollen from velvet grass, perennial ryegrass, Kentucky bluegrass, Timothy grass, rye (Secale cereale), tall fescue (Festuca elatior), field brome (Bromus arvensis), and barley (Hordeum sativum) (20). In another study, monoclonal antibodies to Phl p 4 from Timothy grass also bound strongly to Group 4 homologs in velvet grass, orchard grass, perennial ryegrass, and Kentucky bluegrass (13).

In a study of four patients with chronic cough, positive allergenicity to a 15 kDa excretory-secretory protein from Acanthamoeba was closely related to allergenicity to several species including velvet grass, willow tree, poplar, elm, oak, and cockroach (21). A study in the Cape Peninsula (South Africa) recorded positive radioallergosorbent test (RAST) to Timothy grass in 90% of grass-sensitive patients, even though Timothy grass does not occur in that location. This study suggested the responses probably represented cross-reactivity with other Pooideae grasses which do grow in that area, such as H. lanatus, L. perenne, annual meadow grass (Poa annua), sweet vernal grass (Anthoxanthum odoratum), cat grass (Avena sativa), and reed canary grass (Phalaris arundinacea) (5). An earlier study in Sweden reported that more than 85% of a cohort of 116 individuals with seasonal allergic rhinitis and positive skin reactions to H. lanatus (reported as ‘Yorkshire fog’ by the authors) were also positive to perennial ryegrass and Timothy grass (8).