Abstract--Cardiac remodeling in response to pressure overload involves reorganization of the myocytes and extracellular matrix (ECM). Neurohormonal pathways have been described as effector pathways in left ventricular ECM reorganization in response to pressure overload; we now are assessing the role of the T lymphocyte in this process. Mice with defined differences in T-lymphocyte function (C57BL/6 SCID, C57BL/6 WT, and BALB/c) were treated with 50 mg/L of N^G-nitro-L-arginine methyl ester in their drinking water for 30 days. The immune function of C57BL/6 WT mice was T-helper type 1 (TH1), BALB/c was TH2, and C57BL/6 SCID was null. The arterial blood pressure increased by 30% in all of the strains of mice. However, ventricular stiffness significantly decreased in the C57 SCID, significantly increased in the BALB/c, and did not change in the C57 WT. The characterization of matrix metalloproteinase induction and activation on day 30 was associated with T-lymphocyte function. The total cardiac fibrillar collagen, percentage of fibrillar collagen cross-linking, and the activity of the cross-linking enzyme lysyl oxidase-like-3 (LOXL-3) significantly decreased in the C57 SCID, significantly increased in the BALB/c, and did not change in the C57 WT. This study revealed that the LOXL-3 pathway, namely, gene expression, enzymatic activities, and LOXL-3-mediated collagen cross-linking, was associated with ventricular stiffness and incongruence with lymphocyte function. These data support the concept that the T lymphocytes may play a fundamental regulatory role in cardiac ECM composition through modulation of collagen synthesis, degradation, and cross-linking.