Mapping and characterization of the recessive leaf rust resistance gene Lr83 on wheat chromosome arm 1DS

KEY MESSAGE

The leaf rust resistance gene in Thatcher wheat derivative 78-1 was mapped to chromosome 1DS with SNP markers and designated as Lr83. 'Thatcher' wheat near isogenic line RL6149, a putative derivative of Triticum dicoccoides, was previously determined to carry leaf rust resistance gene Lr64 on chromosome arm 6AL and a second gene temporarily named LrX on chromosome arm 1DS. The objective of this study was to map and characterize LrX in a population of recombinant inbred lines (RILs) that segregated for a single gene. Thatcher line 78-1 with LrX was crossed with Thatcher and individual F₂ seedlings and F₆ RILs were evaluated for leaf rust response. The 208 F₂ plants segregated for a single recessive gene and 148 F₆ lines for a single gene. The RILs and parents were characterized by genotyping by sequencing (GBS). Six GBS markers and five Kompetitive Allele-Specific PCR (KASP) markers were used to map LrX on the distal region of chromosome arm 1DS. LrX was 1 centiMorgan (cM) proximal to marker K-IWB38437 and 0.4 cM distal to GBS marker 1D_9037138. Line 78-1 was crossed with Thatcher wheat lines with Lr21, Lr42, and Lr60 for allelism tests. LrX mapped 19.49 cM from Lr21 and 11.93 cM from Lr42. In the cross of line 78-1 with the Thatcher line with Lr60, one recombinant in 1,003 F₂ plants was found. LrX and Lr60 are at tightly linked loci on the distal region of chromosome arm 1DS. The gene in line 78-1 was designated as Lr83. Cytological examination of RL6149 provided no evidence of transfer of a chromosome segment of an A- or B-genome chromosome to chromosome 1D.

Abstract P3-05-07: Improving CAR T cell function by reversing the immunosuppressive tumor environment of breast cancer

Adoptive transfer of T cells redirected to tumor-associated antigens (TAAs) by expression of chimeric antigen receptors (CARs) can produce tumor responses, even in patients with resistant malignancies. To target breast cancer, we generated T cells expressing a CAR directed to the TAA mucin-1 (MUC1). T cells expressing this CAR (86±1.9%, n=5) specifically killed MUC1-expressing cells (MDA-MB-468 – 45.9±7.3%, MCF-7 – 36.8±3.6) but not MUC1(-) 293T cells (3.7±1.6% specific lysis, 20:1 E:T, n=3). Although these CAR T cells had potent anti-tumor activity against breast cancer cells, when exposed to the Th2-polarizing cytokine IL4 [which is upregulated in tumor samples (Oncomine, p<0.05)] we observed a dramatic reduction in their cytolytic potential [IL2 - 45.9±7.3% vs IL4 - 11.3±3.7% specific lysis, 20:1 E:T ratio, n=4]. Thus, to protect our CAR.MUC1 T cells from the negative influences of IL4, we generated an inverted cytokine receptor (ICR) in which the IL4 receptor exodomain was fused to the IL7 receptor endodomain (4/7 ICR). Transgenic expression of this molecule in CAR.MUC1 T cells (55±4.8% double positive cells, n=5), restored the cytolytic function of CAR T cells (30.9±8.1% specific lysis, 20:1 E:T, n=3). Next, to determine the long term effects of this modification we co-cultured transgenic T cells with MUC1+ tumor cells and measured tumor and T cells numbers. In the presence of IL4, only double positive (CAR.MUC1-4/7) T cells expanded and eliminated the tumors in vitro and in vivo. However, upon tumor elimination, transgenic T cells rapidly contracted, demonstrating the antigen- and cytokine-dependence of the product. In conclusion, CAR.MUC1-4/7 T cells can effectively target breast cancer cells and retain their cytotoxic function even in the IL4-rich tumor microenvironment.

Citation Format: Bajgain P, Tawinwung S, Watanabe N, Sukumaran S, Anurathapan U, Heslop HE, Rooney CM, Brenner M, Leen AM, Vera JF. Improving CAR T cell function by reversing the immunosuppressive tumor environment of breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-05-07.

Association mapping of North American spring wheat breeding germplasm reveals loci conferring resistance to Ug99 and other African stem rust races

BACKGROUND

The recently identified Puccinia graminis f. sp. tritici (Pgt) race TTKSK (Ug99) poses a severe threat to global wheat production because of its broad virulence on several widely deployed resistance genes. Additional virulences have been detected in the Ug99 group of races, and the spread of this race group has been documented across wheat growing regions in Africa, the Middle East (Yemen), and West Asia (Iran). Other broadly virulent Pgt races, such as TRTTF and TKTTF, present further difficulties in maintaining abundant genetic resistance for their effective use in wheat breeding against this destructive fungal disease of wheat. In an effort to identify loci conferring resistance to these races, a genome-wide association study was carried out on a panel of 250 spring wheat breeding lines from the International Maize and Wheat Improvement Center (CIMMYT), six wheat breeding programs in the United States and three wheat breeding programs in Canada.

RESULTS

The lines included in this study were grouped into two major clusters, based on the results of principal component analysis using 23,976 SNP markers. Upon screening for adult plant resistance (APR) to Ug99 during 2013 and 2014 in artificial stem rust screening nurseries at Njoro, Kenya and at Debre Zeit, Ethiopia, several wheat lines were found to exhibit APR. The lines were also screened for resistance at the seedling stage against races TTKSK, TRTTF, and TKTTF at USDA-ARS Cereal Disease Laboratory in St. Paul, Minnesota; and only 9 of the 250 lines displayed seedling resistance to all the races. Using a mixed linear model, 27 SNP markers associated with APR against Ug99 were detected, including markers linked with the known APR gene Sr2. Using the same model, 23, 86, and 111 SNP markers associated with seedling resistance against races TTKSK, TRTTF, and TKTTF were identified, respectively. These included markers linked to the genes Sr8a and Sr11 providing seedling resistance to races TRTTF and TKTTF, respectively. We also identified putatively novel Sr resistance genes on chromosomes 3B, 4D, 5A, 5B, 6A, 7A, and 7B.

CONCLUSION

Our results demonstrate that the North American wheat breeding lines have several resistance loci that provide APR and seedling resistance to highly virulent Pgt races. Using the resistant lines and the SNP markers identified in this study, marker-assisted resistance breeding can assist in development of varieties with elevated levels of resistance to virulent stem rust races including TTKSK.