Example
ClientContext clientContext = new ClientContext(siteUrl);
SP.List oList = clientContext.Web.Lists.GetByTitle("MyList");
int itemId = 2;
ListItem oListItem = oList.Items.GetById(itemId);
oListItem.BreakRoleInheritance(true);
User oUser = clientContext.Web.SiteUsers.GetByLoginName(@"DOMAIN\alias");
oListItem.RoleAssignments.GetByPrincipal(oUser).DeleteObject();
RoleDefinitionBindingCollection collRollDefinitionBinding = new RoleDefinitionBindingCollection(clientContext);
collRollDefinitionBinding.Add(clientContext.Web.RoleDefinitions.GetByType(RoleType.Reader));
oListItem.RoleAssignments.Add(oUser, collRollDefinitionBinding);
clientContext.ExecuteQuery();
