Xcyte Therapies, Inc. Announces Publication of Paper Describing Mechanism of Action of Xcellerated T Cells in Chronic Lymphocytic Leukemia.

SEATTLE -- Xcyte Therapies, Inc. ("Xcyte") (Nasdaq:XCYT) announced today the publication of a scientific paper describing the mechanism of action of its lead product, Xcellerated T Cells, in chronic lymphocytic leukemia (CLL). The paper, entitled "In Vitro Engagement of CD3 and CD28 Corrects T Cell Defects in Chronic Lymphocytic Leukemia", appeared in the February 15, 2005, issue of The Journal of Immunology.

"In patients with CLL, defects in T cells limit their ability to recognize and eliminate the leukemic B cells," noted Mark Bonyhadi, Ph.D., Vice President of Research, Xcyte Therapies, and lead author of the paper. "In this paper, we demonstrate that Xcyte's Xcellerate Process restores the ability of T cells to interact with the malignant B cells and to eliminate them by inducing programmed cell death, or apoptosis."

In the Xcellerate Process, T cells are activated ex vivo using Xcyte(TM) Dynabeads(R), which are superparamagnetic beads to which anti-CD3 and anti-CD28 monoclonal antibodies have been covalently linked. This results in the upregulation of several receptors on the surface of the T cells. In healthy individuals, T cells and B cells interact with each other through receptors on their cell surfaces. In CLL patients, these receptors are poorly expressed, which prevents these interactions from taking place. This may contribute to the growth of the leukemic B cells. Upon activating T cells from CLL patients with the Xcellerate Process, Dr. Bonyhadi and colleagues demonstrated that they could markedly increase the interaction of each patient's T cells with that patient's leukemic B cells. Further studies showed that this enhanced interaction was a direct result of upregulation of key receptors on the patients' T cells, which stimulated increased expression of receptors on the patients' leukemic B cells. The authors reported that these interactions induced the expression of high levels of a molecule known as Fas on the leukemic B cells. Fas plays a critical role in programmed cell death, which was demonstrated to take place in leukemic B cells when mixed with the patients' activated T cells.

"These results are important in understanding the therapeutic effects we have seen in our Phase I/II trial of Xcellerated T Cells in patients with CLL," said Ron Berenson, M.D., President and Chief Executive Officer of Xcyte Therapies. "In that trial, we have observed a 50% or greater decrease in the size of involved lymph nodes in 12 of 17 patients, and a 50% or greater decrease in the size of the spleen in 11 of the 13 patients with an enlarged spleen. We believe the combination of these clinical results and our understanding of the mechanism of action provides a solid rationale for moving Xcellerated T Cells into an advanced phase trial in CLL, and we are working hard to initiate that trial as soon as possible."

Note: Certain of the statements made in this press release are forward-looking, such as those, among others, relating to the therapeutic potential of Xcellerated T Cells and to the ability of Xcyte to initiate an additional trial of Xcellerated T Cells in CLL. Actual results or developments may differ materially from those projected or implied in these forward-looking statements. Factors that may cause such a difference include risks related to adverse clinical results as our product candidates move into and advance in clinical trials, risks inherent in early-stage development and failure by Xcyte Therapies to reach agreement with regulatory authorities regarding trial design. Results obtained in early stage clinical trials may not be predictive of results obtained in larger trials intended to demonstrate the safety and efficacy of Xcellerated T Cells. More information about the risks and uncertainties faced by Xcyte Therapies is contained in our filings with the Securities and Exchange Commission. Xcyte Therapies disclaims any intention or obligation to update or revise any forward-looking statement, whether as a result of new information, future events or otherwise.

Xcyte Therapies, Inc. is a biotechnology company developing a new class of therapeutic products designed to enhance the body's natural immune responses to treat cancer, infectious diseases and other medical conditions associated with weakened immune systems. Xcyte derives its therapeutic products from a patient's own T cells, which are cells of the immune system that orchestrate immune responses and can detect and eliminate cancer cells and infected cells in the body. Xcyte uses its patented and proprietary Xcellerate(TM) Technology to generate activated T cells, called Xcellerated T Cells(TM), from blood that is collected from the patient. Activated T cells are T cells that have been stimulated to carry out immune functions. The Xcellerate(TM) Technology is designed to rapidly activate and expand the number of the patient's T cells outside of the body. Xcyte is currently conducting clinical trials of Xcellerated T Cells(TM) in patients with chronic lymphocytic leukemia, non-Hodgkin's lymphoma, and multiple myeloma.

Xcyte(TM), Xcyte Therapies(TM), Xcellerate(TM) and Xcellerated T Cells(TM) are trademarks of Xcyte Therapies, Inc. Dynabeads(R) is a registered trademark of Dynal Biotech.