Page 1162 - Veterinary Immunology, 10th Edition

reducing expression of CD3ζ. VEGF promotes MDSC production
VetBooks.ir by blocking dendritic cell maturation. IL-1β also promotes MDSC

production. Some MDSCs promote the production of Treg cells.
MDSCs may also enhance cancer cell survival by promoting a

switch from M1 to M2 cells. Collectively, therefore, these cells
effectively suppress immune defenses against tumors. Myeloid
MDSCs have been identified in tumor-bearing dogs where they
suppress lymphocyte proliferation. For example,

immunosuppression has been well documented in dogs with
mammary carcinomas. Affected dogs had normal numbers of T
cells but the proportion of Treg cells was increased as were the
number of MDSCs. These cells increased significantly in late-stage

disease and in dogs with confirmed metastases. It is of interest to
note that MDSCs are also present in corneal allografts, where they
promote graft survival as well as in the placenta where they
prevent fetal rejection (Chapter 34).

Blocking Antibodies

Passively administered serum from tumor-bearing animals may
permit the tumors in recipient animals to grow even faster, a
phenomenon called enhancement. This serum may also inhibit T

cell cytotoxicity. Many tumors release soluble antigens into the
bloodstream, and these may bind to cytotoxic T cells, saturating
their antigen receptors and blocking their ability to bind to target

cells. Alternatively, blocking antibodies may be produced. These
are non-complement–activating, anti-tumor antibodies that mask
tumor antigens on cell surfaces and protect them from attack by
cytotoxic T cells. In general, the presence or absence of blocking
antibodies correlates well with the state of progression of a tumor.

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