Clinical and Experimental Allergy, 1993, Volume 23, pages 661-668 Lymphocyte transformation test with house dust mite {Dermatophagoides pteronyssinus) in normal children, asthmatic children and asthmatic children receiving hyposensitization H. P. VAN BEVER, C. H. BRIDTS, M. M. MOENS, T. E. DE RUCK, A. V. MERTENS, L. S. DE CLERCK and W. J. STEVENS Department of Immunology - Rheumatology, University Hospital Antwerp, Belgium Summary In the first part of this study the proliferative response of lymphocytes (lymphocyte transformation test) to house dust mite (HDM) stimulation in cultures was studied in normal children («= 16), asthmatic children who never received hyposensitization (HS) (n = 50) and asthmatic children receiving HS with HDM for at least 6 months (« = 20). The results are expressed as disintegrations per minute (d.p.m.) and as stimulation index (SI = d.p.m. in the presence ofthe allergen/d.p.m. in the control culture). A positive SI (> 2) was found in 54% ofthe asthmatic children who never received HS, in 30% ofthe asthmatics receiving HS and in none of the normal children. Furthermore, between asthmatics with and without HS, the SI was not statistically different, although asthmatics without HS tended to have a higher SI (median value: 2-13 vs 1 38) {P = Q\ 0). In a second series of experiments the effect of adding interleukin-2 (IL-2) to the lymphocyte cell culture was studied in asthmatic children with and without HS. Interleukin-2 induced an additional stimulatory effect on the lymphoproliferative response to HDM and to phytohaemagglutinin in patients who never received HS, but had no effect in patients receiving HS. We conclude that HS treatment seems to have an inhibiting effect upon this proliferative response, not only inhibiting the degree of the allergen-induced lymphocyte proliferation, but also inhibiting the sensitivity of proliferating lymphocytes for IL-2. These inhibiting effects upon lymphocytic activation could be responsible for the anti-inflammatory effects (i.e. suppression of the late asthmatic reaction) of HS. Clinieal and Experimental Allergy, Vol. 23, pp. 661-668. Submitted 12 November 1992; revised 20 February 1993; accepted 23 February 1993. Introduction Several studies have shown that allergic sensitization to house dust mite (HDM) {Dermatophagoides pteronyssi- nus) is the most common allergy in asthmatics in most parts of the world, affecting between 60 and 85% of patients as compared with 5-15% in control groups [1-5]. Moreover, there is evidence from avoidance studies and from bronchial provocation studies that HDM allergens are a major cause of bronchial hyperreactivity and asthma [6-8]. Correspondence: Dr H. P. Van Bever, Department of Immunology- Rheumatology, Universiteitsplein 1, 2610 Wilrijk, Antwerp, Belgium. Several studies have demonstrated a specific response of lymphocytes, refiecting T-cell sensitization, from HDM-sensitive subjects to allergens from HDM [9-17]. However, the specificity of this type of response is still a matter of debate [18-20]. Recently, a major development has been the isolation of mite-specific T-cell lines or clones by several groups [21]. Furthermore, the effect of hypo- sensitization (HS) with an allergen upon the in vitro lymphocytic transformation showed confiicting results [12,19]. Nevertheless, further study of this in vitro test could help the understanding of the underlying mechan- isms of allergic asthma, as the regulatory function of T- lymphocytes in chronic asthma and in the late phase 661