Summary

Holcus lanatus is a perennial, hairy, tufted grass that is common in temperate regions worldwide. High percentages of individuals (>77%) with seasonal allergic rhinitis have shown positive reactions to cutaneous provocation with H. lanatus pollen. Four allergenic molecules and epitopes have been characterized from H. lanatus pollen, including a Group 1 beta-expansin (Hol l 1), a Group 5 ribonuclease (Hol l 5), a Group 4 berberine bridge enzyme (Hol l 4), and a Group 2 protein with unknown biological function (Hol l 2). Hol l 1, Hol l 5, and to a lesser extent Hol l 4 have shown a high degree of cross-reactivity with homologs from other Pooideae subfamily grasses.

Route Of Exposure

Inhalation of pollen from velvet grass can induce respiratory and cutaneous symptoms in sensitized individuals

Clinical Relevance

Pollen from velvet grass can induce respiratory and cutaneous symptoms in sensitized individuals such as allergic rhinitis, or hay fever. A study in Berlin (Germany) demonstrated a highly significant relationship between daily concentrations of pollen species including H. lanatus and rhinitis symptoms of grass pollen allergy sufferers (p<0.016). In Costa Rica, approximately 3 in 4 (77.7%) of 184 patients with allergic rhinitis showed positive skin prick tests to H. lanatus pollen. In Sweden, 89% of 133 individuals with seasonal allergic rhinitis had a positive intracutaneous reaction to H. lanatus grass pollen (referred to by the authors as ‘Yorkshire fog’).

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.

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); 95% homology and 93% identity with Phl p 1 in an analysis of the surface topographies of the allergens; and 88.1% sequence identity with Pha a 1. Approximately 70% similarity has been demonstrated between Hol l 1 and Zea m 1 (maize, Zea mays) of the Poaceae subfamily Panicoideae, however no antigenic cross-reactivity was seen between Hol l 1 and Cyn d 1 (Bermuda grass, Cynodon dactylon) of the subfamily Chloridoideae.

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). 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. 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.

More than 75% of grass pollen allergic patients produce specific IgE antibodies against Group 4 allergens. 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). 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.

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. 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). 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.