Major histocompatibility complex class II expression on CD14 ^pos cells in canine peripheral blood across various hematologic disorders—a preliminary study

Monocytes are key cells of the immune system; various cell subsets have distinct phenotypic characteristics and functions. We characterized canine monocyte subsets in different hematologic disorders by analyzing peripheral blood cells for their expression of CD14 and major histocompatibility complex class II (MHCII). Flow cytometry data of canine blood samples were evaluated retrospectively. We assigned 132 total cases to one of the following categories: healthy, inflammatory, T-zone lymphoma/leukemia (TZL), T-cell chronic lymphocytic leukemia (CLL), B-cell CLL, T-cell stage V lymphoma (SVL), B-cell SVL, acute lymphoblastic or undifferentiated leukemia (ALL/AUL), or acute myeloid leukemia (AML). We analyzed age, sex, breed, light scatter properties, WBC count, CD14^pos cells, CD14^posMHCII^pos cells, CD14^posMHCII^neg cells, and the CD14^posMHCII^pos/CD14^posMHCII^neg ratio. All groups were compared with the healthy group. In healthy dogs, CD14^posMHCII^pos cells were larger and had a more complex internal structure than CD14^posMHCII^neg cells. Absolute numbers of CD14^posMHCII^neg cells were significantly increased in all groups except the TZL and B-cell SVL groups. Absolute numbers of CD14^posMHCII^pos cells were significantly increased in the T-cell CLL, B-cell CLL, and T-cell SVL groups, mildly increased in the inflammatory and TZL groups, and decreased in the ALL/AUL group. The CD14^posMHCII^pos/CD14^posMHCII^neg ratio was markedly decreased in both acute leukemia groups, being <1 in all cases and usually close to 0. We found significant changes in canine monocyte subsets in different hematologic conditions. To our knowledge, a decreased CD14^posMHCII^pos/CD14^posMHCII^neg ratio has not been identified previously as a hallmark of acute leukemia in dogs.